This is one of my favorite subclasses. An undead warlock patron has always felt like a no-brainer, and after the really underwhelming Undying patron from SCAG (which wasn't even necessarily undead) the true, full Undead patron that we got in Van Richten's became one of my favorite Warlock subclasses instantly.
Warlock subclasses got one major change in 5.5, giving them all of the spells associated with their subclass rather than merely expanding their options. Notably, this does also mean that sometimes your Patron spells will be spells you could have taken anyway.
Many a Darklord could serve quite well as an Undead patron, from Strahd to Azalin to Lord Soth. So, it's very much a good choice for Ravenloft, but I think it works in basically any D&D setting quite well.
Let's get into it:
Undead Spells:
1st: Bane, Rack of Sickness
2nd: Blindness/Deafness, Phantasmal Force
3rd: Speak with Dead, Summon Undead
4th: Greater Invisibility, Phantasmal Killer
5th: Antilife Shell, Cloudkill
Naturally, Warlocks need to use spells that scale well if they're natively 1st-to-4th level. Greater Invisibilty sadly doesn't upcast. Summon Undead is probably the go-to concentration spell for us (having used it a lot on my Wizard, the 5.5 update for it has made the Ghostly and Putrid options quite powerful, as long as foes aren't immune to fear or poison, respectively).
Level 3:
Form of Dread:
As a bonus action, you can beocme an avatar of your patron, gaining the following benefits for 1 minute or until you are incapacitated or end the form voluntarily. You can do this Cha times per long ret. The benefits are:
Facsimile of Life: You gain temp HP equal to 1d10 plus your Warlock level.
Fearless Form: You have immunity to the Frightened condition and end it if you are frightened when you activate the form.
Frightful Avatar: Once per turn, when you hit a creature with an attack, you can force them to make a Wisdom saving throw or be frightened until the end of your next turn.
I think this is totally unchanged (maybe it was PB times per Long Rest in Van Richten's) but it's a solid option that should be available most combat unless you're in a real grueling dungeon-crawling campaign.
Level 6:
Grave Touched:
You get the following:
Arcane Necrosis causes Necrotic damage you deal to ignore resistance. Also, once per turn, if you cast a spell that deals damage, you can make that spell's damage Necrotic.
Once again, I think this is unchanged. Given that Warlocks have Force as their main damage type, this on its own isn't going to often be much of a benefit. Also, I think very few monsters have resistance rather than full immunity to Necrotic.
Dreaded Necrosis allows you, when you hit with an attack that deals Necrotic damage while you're using your Form of Dread, you can roll one additional damage die once per turn.
As a reminder, the new Pact of the Blade allows your Pact Weapon to deal Necrotic, Psychic, or Radiant. So this should work for Bladelocks as well. This is the main reason to use Arcane Necrosis, even on a reliable spell like Eldritch Blast, unless you're fighting ghosts or something. Basically, turn one of those hits into a crit.
Undead Endurance prevents you from gaining exhaustion from dehydration, malnutrition, or suffocation, and you don't need to sleep and can't be magically put to sleep.
So, no worries about food or drink, and you don't have to breathe. Depending on the campaign, this might not come up or it could be quite helpful. (Notably, I made a Reborn Undead Warlock for a very brief West Marches campaign, and so there's a little redundancy there).
Level 10:
Necrotic Husk:
You get the following:
Necrotic Resilience gives you resistance to Necrotic damage, and this becomes full immunity if you're in your Form of Dread.
Unholy Resuscitation allows you to cause an explosion of deathly energy if you drop to 0 HP. Each creature in a 30-foot emanation of your choice makes a Con save, taking 2d10+Cha necrotic damage on a failure or half on a success, and then your HP becomes twice your Warlock level and you gain 1 level of Exhaustion. You can use this once per short or long rest.
Couple things: first off, you can use this far more frequently than before. The old version required you to finish 1d4 long rests, one of the rare features that didn't necessarily come back the next day. That's a buff. The potential very subtle nerf is that it doesn't say if you "would" go to 0 HP. Instead, it's if you do, then these things happen. Thus, technically, in a strict reading, you do momentarily go unconscious, which means that you'd drop concentration and Form of Dread if that happened.
As a DM, I'd probably rule that this should work like other features that allow you to avoid going down, but it's a weird little wrinkle in there.
Level 14:
Superior Dread:
Your Form of Dread is improved in the following ways:
Dread Resistance gives you resistance to bludgeoning, piercing, and slashing damage.
Ghostly Flight gives you a fly speed equal to your speed (and you can hover). You can also move through creatures and objects as if they were difficult terrain, taking 1d10 force damage if you end your turn inside one (is that here in, like, all of these Horrors Within subclasses?)
Profane Casting lets you cast Conjuration and Necromancy Warlock spells without any spell components unless there are costly or consumed ones.
This is the only really huge change to the old version, replacing Spirit Projection. The old feature required you to park your physical body somewhere out of the way while you spirit gained these benefits. This is more elegant, but not as funky. It also lasts shorter, as the old Spirit Projection lasted an hour.
But, this is probably better, as you're probably using Form of Dread anyway, and the old version of this could only be used once per long rest.
Overall Thoughts:
Up until level 14, this feels very much like a slight tune-up and update rather than a significant redesign, and honestly, even Superior Dread is largely just refining the existing feature.
But that's fine, because Undead Warlocks were already a solid subclass. Given how Warlock subclasses have changed in 5.5, I'm happy to have a revision for this one - I'd honestly thought it would have been a good choice for the Player's Handbook.
Anyway, that brings us to the end of the Horrors Within subclass review. I'm going to look at some other things, like Species, Dark Gifts, and probably go over a few of the monsters as well.