Demi-Legendary Creatures - Creating a Phased Fight That's Exciting and Cool

Ok, DMs:

Player characters are powerful. It's the nature of the game, and you want your players to feel strong (well, unless you're playing a horror-based campaign... I have some ideas about that, like just balancing encounters as if they were all X levels higher or if there were X more players in the party, but that's another post and not based on experimentation) and heroic. And after all, you want them to ultimately prevail against the monsters you throw at them.

But frankly, I think that most enemies - especially if you run games with big groups - go down so insanely quickly that you, as DM, don't even get to use their fancy toys.

Generally speaking, I think that any time the players outnumber the monsters you throw at them, if there's one particularly central figure to the fight (and basically always if you ever have them fight just one monster) you'll want to make the monster legendary.

First, I recommend giving them max health - just look at their hit dice and assume they rolled the maximum for each die.

If your party is very heavy on magical crowd-control, give them 3 daily legendary resistances. But most importantly, to solve action economy issues, give them legendary actions.

I recommend the following:

Let them make one of their regular attacks (if it's a spellcaster, let them cast a cantrip) this can often just be a single legendary action if one attack isn't that big - most dangerous monsters have multiattack - but if your monster only makes one or two attacks, but these attacks do massive damage, consider making this cost 2 legendary actions to balance it.

Then, give them some sort of movement ability that doesn't provoke opportunity attacks - a half-speed movement with no opp attacks is a pretty standard one I use.

And then you might come up with something creative - maybe an area-effect around them. Keep the damage lower than their typical attacks, as this can hit multiple party members and thus feel a lot bigger overall (you can make this cost extra actions if you want to buff its damage.)

Now, this works pretty well for any creature you want to make a big boss monster. (In general, this will still make the encounter fairly balanced for the party - if you really want to amp it up be very difficult, consider doubling the health after maxing it.)

But let's say you're not really looking for an epic, plot-important encounter, and just want to soup up a random encounter or a sidequest.

Here's what I did tonight:

You max the health, but you don't use legendary actions... until they get to the ordinary average health.

Then, the legendaries activate, and for the rest of the fight, things get more difficult.

It essentially creates two phases of the fight - one in which the party is likely to feel pretty confident at first as they wail on the thing, and then it kicks into high gear after they've gotten a few hits in.

My example was the Gearkeeper Construct from Explorer's Guide to Wildemount - something I flavored as an Izzet invention that had been secretly corrupted with Phyrexian oil, and so went berserk.

Its normal health is 161, but maxed out it's 238. So once the party had done at least 77 damage, it got the following legendary actions:

Blade Spin: Creatures within 10 feet of the construct must make a DC 15 Dex save, taking 2d8 slashing damage on a failure, or half on a success.

Bowl Through (Costs 2 Actions): The construct moves up to half its movement speed. Any creature whose space it passes through must make a DC 15 Strength save or be knocked prone and take 3d8 slashing damage. This movement does not provoke opportunity attacks.

Attack: The construct uses its Arm Blade or Spear Launcher attack.

The fight (immediately following a fight with some Fluxcharger Weirds and Iron Defenders) only lasted two rounds against a party of 6 level 10 players, but there was enough to do that it felt reasonably epic.