I learned a harsh lesson early on in my tenure as a DM for D&D. My players had had a couple of short encounters in their adventures, but I had them go to The Tomb of Sed, an ancient ruin dedicated to a no-longer-worshipped deity (the truth was that Sed was actually an angelic servant of one of the real gods who had not yet revealed herself in that era,) which was a small dungeon with basically a trap, some Shadows, and a Spectator waiting for them as the dungeon boss.
Spectators are CR 3, and the party was, I think, either level 2 or 3 at this point (they were XP leveling, and with the undertuned encounters from the 2014 DMG, leveling was a bit slow).
However, when they got to the final room, a sort of concave inverted step pyramid, with the Spectator hovering near the bottom, the Fighter and Paladin both beat it initiative and killed it before it could get a turn, with a lucky smite-crit sealing the deal.
Spectators are odd because they're a little on the complex side to be just a minion, but they're not legendary creatures like Beholders (they're also not assumed to be evil-aligned).
Gaming tradition holds a very popular trope: the Boss. I don't know exactly where the term originated, but it makes a certain amount of sense - the biggest, toughest enemy is in charge of all the little minions you've been fighting leading up to that point, even if, narratively, that's not really what's happening.
To define what a boss is in games, I guess we should narrow it down to various points:
They are a tougher enemy that requires more time, effort, and strategy than a normal enemy.
They are typically unique, or at least rare compared to other monster types (this depends a lot on genre - some games have far too many levels/areas for every boss to be totally unique, though repeating bosses are sometimes considered more mini-bosses).
They tend to come at the end of a level/dungeon, or at least after a significant stretch of non-boss enemies. (There are exceptions here when they want to subvert an idea, like Phantoon in Super Metroid, whom you fight first before all the enemies on the Wrecked Ship activate.)
They are often fought alone, or if fought among other enemies, they are by far the biggest threat. (Again, there are exceptions here, with dual-bosses like Ornstein and Smough from Dark Souls, where the biggest part of the challenge is that you have to fight two bosses at the same time.)
Both D&D and Draw Steel - the systems that are clearly doing the most to inspire Deathblow's mechanics - have ways of doing Bosses.
In D&D, these are Legendary creatures. Legendary creatures have two explicit design elements that other monsters don't. First, and probably most impactful, is Legendary Actions. Ordinarily, a monster can only do anything when it's not their turn using a single reaction. Legendary Actions give them three opportunities to do something in between players' (or other monsters') turns. Next, they have Legendary Resistance, which allows these boss-like monsters to automatically succeed on saving throws. I think the intent here is primarily to avoid crowd-control abilities and spells that would end the fight immediately, like Banishment. A third aspect of Legendary monsters in 5.5 that doesn't get as much of an explicit call-out is that they tend to have either proficiency or, at higher levels, expertise in Initiative. This makes it far more likely that the monster gets to act first.
Draw Steel approaches things somewhat similarly, but also tends to be more explicit in how a boss is distinct from other monsters. Leaders are designed to be those fights with minions, but Solo monsters have several features that try to make them serious, epic threats.
For Deathblow, the focus on combat would be high-stakes boss fights, effectively. A bit like Shadow of the Colossus, this would be a game in which (nearly) all combat is against singular, memorable, epic monsters.
That means that every "headliner" monster, which would be the majority of those found in any monster book the game might have, would need to be cool, unique, and interesting.
Monster design is a tricky and subtle thing: I saw a sneak preview of Cthulhu's stat block coming in the upcoming Ravenloft book, and initially I was underwhelmed, as he just kind of has a grab attack and then something that can deal psychic damage to grappled creatures, along with some teleportation abilities. But then I saw that in his spellcasting trait, he can cast both Dream and Geas, and can target creatures with Geas while invading their dreams. This is... well, it's pretty Cthulhu, isn't it? And it creates some interesting opportunities for gameplay before Initiative is rolled (I'd have to probably run him to see how he feels to play actually in combat).
Still, broadly speaking, boss monsters need to overcome the problems with the action economy. If your boss monster is outnumbered by the party (which should be the case every time) they run into this problem where the party just has more opportunities to do things than they do. The party can respond with many different things to each act that the monster performs.
Notably, I think that something like Multiattack among D&D monsters doesn't really solve this - Multiattack tends to commit you to doing one major "verb," as in "attacking," and while they might split their attacks between targets, chances are that they're all coming toward a single PC, so it's actually not all that different from just one big attack.
Legendary actions, thus, are a big part of fixing this. But Legendary Actions are also often limited. In 5.5, typically you get two or three Legendary Action options, one of which is a standard attack while the other one or two are going to be more flashy things that might involve movement or imposing conditions on PCs, but can only be used once per round.
Draw Steel has rough equivalences to a lot of 5E tech: Legendary Resistance is replaced with a feature that lets the boss pay health to end conditions on it - another way of eliminating the "null result" and giving players a consolation prize for imposing conditions. Villain Actions are like Legendary Actions, but each only gets used once per encounter, and they tend to be bigger and flashier because of this.
But I think the really fascinating bit of tech in Draw Steel's solo monsters is that they get multiple turns per round. In Draw Steel, there's no set turn order determined by individual initiative rolls - instead, the party and the monsters alternate turns (with weaker monsters getting to act in squads on the same turn). Solo Monsters get to take two turns a round, and just need to let at least one PC go between their turns.
This, naturally, lets the monster do a lot more in a fight because they're literally getting twice as many turns (unless they get killed before they take their second turn of a round).
However, I wonder if we could take this further.
In Daggerheart - a system I am admittedly far less familiar with - there is no established initiative whatsoever. Instead, players can take turns until something causes the "spotlight" to revert to the GM. I know that this happens when someone rolls with Fear (which happens roughly though a little less than 50% of the time - now I want to figure that out mathematically,) but the GM can also spend Fear to take the spotlight. I also think that in Age of Umbra they might also get it when an attack misses, though I might have misunderstood that.
The point is, in that game, the monsters can potentially act far more often, and I get the sense/vibes that the game was designed to make individual monsters far more threatening because of this.
So, what if we did the following:
What if the monster gets a turn after every PC's turn?
The consequence here, which could be good or bad, is that the monster scales significantly with the number of players at the table. If you have a tight band of three Night Hunters, the monster gets three turns per round. If you have a hefty squad of 6 players, the monster gets six turns.
The good thing here is that the monster naturally has scaling action economy. I don't think we need anything like Legendary Actions or Villain Actions when the monster is constantly on the move. The party is never going to be able to overwhelm the monster with sheer numbers, because the more they bring, the more the monster can fight back.
The bad thing is where that throws all the other elements of scaling. Having a large party will still let you kill the monster in a shorter number of rounds (assuming the Stamina doesn't scale up as well with the party size) but if the monster is getting more turns, that means that damage-per-round on both sides is scaling up by a fair amount, and thus, the target of the monster's attacks is going to take more damage between each of their turns.
To illustrate: a party with a Witch, Warrior, and Assassin is confronting a Banshee. The Banshee has some kind of Death Wail attack that deals, say, an average of 10 psychic damage. The Warrior, whose abilities are probably focused on holding a monster's attention and protecting allies (basically tanking) is getting her full ire. So, on a round, the Warrior is taking all the Banshee's attacks and so can expect to take 30 damage per round if we're using the "monsters act after every turn" approach.
But if the party now consists of the Warrior, Witch, Assassin, and also a Hunter, Inquisitor, and Occultist. The party is putting (on average) twice as much damage out, but the Banshee is now getting twice as many turns, meaning that the Warrior is now taking 60 damage per round, rather than 30.
Is that ok? Are we ok with that?
Because there's a world in which that might be all right - maybe the challenge of playing in a large group of Night Hunters is that you need to be more specialized and coordinated. Not only does the Warrior focus on keeping the Banshee from attacking their allies, but the other players need to use abilities that help keep the Warrior up - the Witch might need to use more healing abilities, and the Hunter might need to use abilities that reduce a monster's damage output or perhaps draw them away (physically) from their target. And perhaps, in a larger group, Warriors (or anyone who takes on the task of tanking the boss, which I could see being something that Inquisitors and maybe Mechanists would be decent at - maybe Assassins could as well, but in more of a "focus on me as I run away" manner) would need to focus more on defensive abilities while in smaller groups they can contribute more to damage.
One of the things I really like about Draw Steel's solo monsters is that they have way more Stamina than lesser monsters of the same level. A Werewolf (one of the two level 1 solo monsters) has 200 Stamina, compared with 26 for a level 1 Platoon creature (platoon being the organization level where you can have roughly one monster per player in the encounter if they're the same level). In other words, if I had four level 1 players in Draw Steel, I could have them fighting four Dwarf Gunners, who would have a total of 104 Stamina, or a Werewolf with a total of 200 Stamina.
While that might seem inappropriately spongey, I actually think it's smart - even with the various action-economy enhancements like the second turn and Villain Actions, it's still not quite matching what four less powerful monsters could do. Having the beefy stamina means that the monster is going to be able to stick around long enough to actually get to do their cool stuff.
If we really want to scale the monsters to the party size, what if they had Stamina based on the size of the party as well?
Again, if we've got this Banshee, maybe she has 50 Stamina per party member, so in that group of 3, she's got 150 Stamina and if it's the larger party, she's got 300.
Now, are we worried about double-dipping? The monster is already doing twice as much damage to the party if we're letting it act after every PC's turn. Now, we're making it last twice as long. Thus, doubling the party effectively quadruples the monster's total damage output, because we can assume it's going to get twice as many turns before it's taken down.
Assuming an average damage output among players - say 15 per turn - we can then assume that regardless of the number of players, the monster should last a certain number of rounds. 3 players doing 15 damage per round each would mean 45 damage per round and thus could put out 150 damage some time in the middle of round 4. 6 players doing 15 damage per round would do 90 damage per round in total, and thus would have hit 300 damage again some time during round 4 on average.
But there are other ways to scale monster damage with a party.
The biggest, most obvious one, is just multi-target damage. If that Death Wail doesn't just hit one Night Hunter, but damages everyone within 60 feet or something, that is probably going to hit the majority of the party, if not everyone. And in that case, the monster is literally dealing more damage the more players there are.
One of the goals I'd have with combat design (which is likely to take a lot of cues from Draw Steel, though I'm going to stick with real-world measurements like feet, even if under the hood it'll really be units of 5 feet that act like "squares" in Draw Steel) is to make sure that creatures don't get locked down in place. In cinematic fights, movement is a huge thing - you almost never see two combatants just standing in the same place (the lightsaber duel in the original Star Wars is notable in how kind of dull it is, which got corrected in Empire Strikes Back with the deadly cat-and-mouse game between Luke and Vader).
Deathblow would eliminate Opportunity Attacks in order to encourage constant re-positioning and use of terrain.
But to get to the point regarding boss design, I think that bosses are probably going to also jump from target to target a lot - indeed, I'm not sure that I'd really design tank-y abilities, or at least taunt-like abilities. Tanks would be built to endure attacks, but I think they won't be able to compel monsters to strike them instead of their allies except by doing things like grappling or otherwise reducing the monsters' movement abilities.
This could, in a weird way, actually benefit the players because if the monster is not going to focus down a single player, the size of the party effectively raises the total Stamina of the party.
That being said, we don't want monster design to rely on GMs playing suboptimally. If the monster is going to be jump from target to target, they'd want an incentive to do so.
And surely, different monsters might act differently. I could imagine a Banshee being evasive and using ranged screams that damage multiple PCs, so the challenge is reaching her and getting your strikes in. A Hexen (again, my vaguely Hag-like equivalent) would probably want to place curses on each of the party members, which might require them to get up close to them sneakily. Maybe the curses scale up in damage as the monster puts more of them on the target. A vampire, on the other hand, probably tries to isolate and exsanguinate individual targets.
Multi-target attacks plus action scaling per player once again double-dips.
So, while it might be the most boring way of doing this, I think that maybe the right call for monster scaling here is to simply have the Stamina scale up with the number of party members. I do think that this should, all in all, actually favor the players because a larger party is covering more bases, and can specialize in ways to tip them over the top - say the Occultist has various ways of boosting the damage of other players through eldritch rites while the Hunter can make the monster more vulnerable with certain attacks, opening up the Assassin to land some insanely high-damage blow that is more than what they would get if they were just each individually trying for their best damage abilities (like a Grave Cleric doing Path to the Grave before a Paladin hits with a Divine Smite in 5E).
Still, we'll want to at least boost the action economy of a monster. I really like Draw Steel's "two turns per round" approach, which is a flat boost rather than a scaling one, but does simplify the math and also allows the monster to mostly adhere to the same action economy rules as the player while still letting them do more things.
If we're really worried about scaling Stamina double-dipping with AoE effects, we could target-cap AoE abilities. I think melee-focused monsters like a werewolf might not be much of an issue - if their "Sweeping Rake" (a hypothetical ability) does slashing damage and maybe puts a bleed on every target within 5 feet of them, that's naturally going to limit it to those characters who are grouped up with them in melee. But our Banshee's Death Wail is going to be a huge radius, and so we might say that it deals its damage to only three targets, so there's a cap on the total damage it can do.
Another idea, if we were to have turns scaling based on the party, is to limit monster resource generation to rounds - the resource (I'm thinking Darkness, though it's really not too different from Malice in Draw Steel) would need to be spent to use a monster's more powerful abilities, and thus those "extra turns" that it gets might feel a bit more like legendary actions, which tend not to give a monster their full multiattack in 5E. The GM could choose to spread out their expense of Darkness over the round to do cheaper abilities, or they could blow it all on one big ability and then spend the other turns doing the monster's basic, weakest attacks.
Resource generation is something we really need to figure out for the game - both how it will work for PCs and how it will work for monsters.
To be fair, I'm getting very theoretical and some of these might need to be ironed out via playtesting.
But I think we need a core concept here that we can apply to our monsters. Monsters in Deathblow need to all feel like big, epic fights, because the whole point is that the adventure is building toward the confrontation.
I think that means we need to A: give them a lot of "action economy slots" to do iconic things. A Vampire ought to be able to grab a creature, drag them off somewhere, and bite them all in one turn, but we also need some opportunities for shapeshifting and disappearing in shadows. And B: we need to give them flashy, memorable, and unique mechanics to distinguish them from the rest.
Players should remember each headlining monster they've fought. Special Deathblow mechanics are certainly one way to make them memorable, but I think building bespoke mechanics for each kind of monster is also really a good idea. Again, looking at Draw Steel's Werewolf, there's a unique "condition" that the Werewolf imposes called Accursed Rage - the longer you fight it, the more likely you are to succumb to a berserker rage and strike your friends (wonderfully, if you're the lycanthrope-adjacent Stormwight subclass for the Fury class, you're immune to this because you're kind of already a were-creature anyway).
To be frank, if this game were to be finished and published, I imagine the monster book would actually be a fair degree thinner than it is for its main inspirations. But I also imagine that a monster stat block would be a lot more complex.