Flavoring The Undead Warlock

 Sword Coast Adventurer's Guide is an odd book - it came out in 2015, not long after the launch of 5th Edition, and is technically a campaign setting book for the Sword Coast - the region of Faerun, itself a continent on Toril, the primary world of the Forgotten Realms setting, that is the most storied and classic of D&D settings (while Greyhawk, being the creation of Gary Gygax himself, could argue for primacy as the "main" D&D setting, through at least 2nd, 3rd, and 5th Edition the Forgotten Realms has been the "standard" setting for the game.) But compared with other campaign setting books, it's weird. It came with several subclasses, some of which were reprinted in Xanathar's Guide to Everything, but others that you might not even remember existing: have you ever seen someone play a Battle Rager Barbarian? A Purple Dragon Knight Fighter? A Way of the Long Death Monk? These are all officially-published 5th Edition subclasses.

Among the oddities of SCAG, there is the Undying Warlock. The general interpretation for this subclass is that your patron is some form of undead, like a Lich. However, the book weirdly does all in its power to avoid explicitly saying your patron is undead. Instead, the Undying is a being who has avoided death in some powerful, magical way. So you could technically have any immortal being - your patron could be some sentient treant suffused with druidic magic that keeps it from ever dying.

This intentional vagueness has always been a little frustrating. It also doesn't help that the Undying Warlock's subclass features are, well, not particularly great. For example, you effectively have a permanent sanctuary spell against undead, which could be great if you're dealing with a bunch of undead, but could be useless in other situations.

The Undead Warlock Patron coming in Van Richten's Guide to Ravenloft will, I think, supplant the Undying in many ways, and I think that's fine. In this case, the patron is explicitly an undead being - a lich, a vampire, a ghost, a death knight - any powerful undead.

By making this explicitly a patron who is necromantically inclined, the flavor of the subclass can be more explicit. Obviously, we don't have the official rules for the subclass just yet, but we did have an Unearthed Arcana article a few months ago. The Undead Warlock can alter much of their damage to become necrotic, and when they do so, they can often deal additional damage that way (one thing I am almost certain will be nerfed is that, as written right now, an Undead Warlock who uses this boost can deal 2d10 necrotic damage with each hit of an Eldritch Blast, which means a tier 4 Warlock could do 8d10 damage with a cantrip without even critting or accounting for agonizing blast.)

Given that the mechanics are in flux, let's talk flavor.

Naturally, Liches, Vampires, and the like are popular D&D monsters. One of the common themes for these sorts of creatures is their ability to come back after being destroyed. A Lich respawns after a couple days near wherever their phylactery is hidden. A vampire turns to mist when it is killed and can potentially escape to its resting place to regenerate. A death knight comes back as long as its soul hasn't been redeemed. And ghosts remain as long as their unfinished business remains unfulfilled.

Undead are usually evil, which means any warlock who has made a pact with an undead patron has bargained with a dangerous entity. There are exceptions, though: the Undying Court in Eberron is made up of undead who are animated by positive energy - this is reflected in their resilience to radiant damage and their vulnerability to necrotic - and are all good-aligned, acting as undead caretakers of their society.

Given that the subclass is coming with Ravenloft (and thus will probably not be AL-legal unless it gets a reprint in another "Everything" book) the most obvious connection here would be to use a domain's darklord as your patron. Not all darklords are undead, but many are - if you look at classic Universal monsters, the likes of Dracula, the Mummy, and Frakenstein all count (even if by D&D mechanics, a flesh golem like Frakenstein is technically a construct. Oh, and before you go "um, actually, Frankenstein is the creator, not the monster," I'll nerd you one better by reminding you that in the book, the creature takes Victor Frankenstein's name, so the Boris Karloff character is, indeed, named Frankenstein.)

Strahd von Zarovich, or other figures like Azalin the Lich, are the rulers/prisoners of their own domains of dread. They are all powerful - imbued with a deep connection to the realm itself - and could certainly make for good warlock patrons. I see two routes for this:

One is that the darklord of the domain you find yourself in is your patron. You could play Curse of Strahd as a warlock of Strahd himself. This is going to introduce some really interesting dynamics: first off, you could play as his secret agent within the party (though this might make it hard to resolve the story.) Alternatively, you might be a rebel against Strahd, one who did gain power from him but now acts against him. Given the transactional nature of a Warlock's pact, it's possible that you made a deal for power and considered that a closed deal - but now you're essentially swept into this conflict with someone you dealt with in the past. If you go this route, you should definitely figure out with your DM if your end of the pact has already been fulfilled, or if it's an ongoing obligation that would allow the darklord to turn your powers off. While mechanically it'll be a lot easier if the deal is done and irrevocable, you can sacrifice some of your own player power for a potentially very rewarding challenge in which you need to deal with the fact that the antagonist can take your power away (perhaps creating a new side quest in which you can rip that power fully from them and make it your own.)

Van Richten's Guide to Ravenloft will be breaking with some of the 2nd Edition lore - the Core, a continent-like landmass that connected the various Domains of Dread will be shattered, which means that travel between domains will at best be a metaphysical jaunt through the Mists, rather than a physical one. Still, I see no reason why the rivalries and scheming between Darklords can't continue (especially in your game,) and thus your character might actually be serving a being like Strahd while you and your party fight off the undead zombie hordes of Falkovnia (which will be what's going on there in 5th Edition.) Warlocks often have a "deal with one evil to defeat another" vibe going with them, and so it could be fun to play as the agent of one against another. Vecna hasn't been in Ravenloft since 2nd Edition, but I sort of love the idea of serving the Whispered One.

And of course, relationships with Warlock patrons can run the gamut of trusted servant or bitter enemy. While some mechanics are a little worrying if you're not on good terms with your patron (like the level 20 feature that allows you to plea with your patron for ten minutes to get your spell slots refunded) the folks at WotC have generally ruled that once a pact is made, the power the Warlock gets is permanent, and that they likely already fulfilled their end of the bargain to get that power. Personally I find it more dramatic when the Warlock still has work to do to pay the patron back, but if you go this "settled" route, you can have a lot of fun playing against a patron who will really regret making that deal with you.

Given the prevalence of undead foes across D&D, this is a subclass I think can very, very easily work in any setting. As I said before, the Obzedat Ghost Council in Ravnica could be a great option for a council-style patron (with perhaps different relationships with each of its members).

Mechanically, assuming things don't change that much from the Unearthed Arcana version (though I really think they'll nerf Form of Dread/Grave Touched combination as that would essentially turn every Eldritch Blast into a crit, and every crit into a double-crit) the Undead Warlock might actually have some difficulty fighting undead, given the focus on necrotic damage, which many corporeal undead are resistant to and to which ghostly undead are typically immune. Still, this is often an optional thing.

One of the interesting aspects of the Undead as a patron is that they used to be mortals. Now, granted, in D&D oftentimes beings like fiends and celestials actually did begin as mortal souls, becoming fiendish or celestial after they died and went on to the Outer Planes, and in fact a lot of the gods at least in the Forgotten Realms were powerful adventurers who ascended to godhood once they amassed enough power. Still, the undead are probably more closely connected to their mortal lives, which gives you an opportunity to potentially give them a lot more of a human personality.

Granted, there's a tendency for undeath to kind of dull the good parts of a personality. Vampires are typically portrayed as having lost all the moral inhibitions of their mortal lives (often in a kind of seductively sensual way, but also in the whole "I'm ok with tearing out other peoples' throats to feed" way) while Liches in the Monster Manual are described as having become obsessive husks of the dynamic minds they were as mortal wizards. Still, I think there's plenty of wiggle room in that (and room for interpretation) that you could have a fun and/or funny relationship with your undead patron in a way that might be harder to do with, say, a Great Old One. (I wrote a character backstory for a warlock who carries his patron's phylactery around because she can no longer manifest corporeally thanks to the wards placed on said phylactery by her literal saint of a brother, and she acts as a bored, mischievous troublemaker/imaginary friend rather than some kind of dark corruptor.)

Unlike Celestials, Fiends, Archfeys, and Genies, Undead patrons likely originate on the Material Plane (outside of those found in Ravenloft) and thus could more often be a real presence in the campaign.

Additionally, given this patron's formerly living status, you could easily have a patron of this sort be an ancestor of your character's. Perhaps a noble family's secret to their power and prestige is that their great-great-great-great-great-great-grandmother is a powerful lich whose lair extends deep below the family manor.

So, ultimately, I think this subclass is filling a niche whose only reason for not existing yet in 5th Edition was the existence of the rather lackluster Undying. The possibilities for such a patron are vast. It's less thematically limited to the Gothic setting of Ravenloft than the College of Spirits Bard (though as I say in the post on those folks, you can easily reimagine them for a different vibe) and so I hope that if 5th Edition gets another setting-agnostic rules expansion like Tasha's Cauldron of Everything that we'll see these reprinted (for those of us who play Adventurer's League.)