Reanimator Artificer

 Behold: the first ever official 5E Artificer subclass to be introduced in a book that doesn't include the whole class. Yes, it's a setting-appropriate horror-themed subclass that lets us channel our Victor Frankenstein (or maybe a little bit of Herbert West?)

A pet subclass somewhat like a Battle Smith, the Reanimator creates an undead abomination that is relatively expendable. As you level up, you'll have different ways to customize your pets, giving you a certain degree of flexibility with the roles they serve. Unlike the Battle Smith, you will be primarily a spellcaster. Let's just dive in:

Reanimator Spells:

Cantrip: Spare the Dying

1st: False Life, Witch Bolt

2nd: Blindness/Deafness, Enhance Ability

3rd: Animate Dead, Lightning Bolt

4th: Blight, Death Ward

5th: Antilife Shell, Raise Dead

    While I'm not a fan of Blight, this is overall a very solid spell list, and while Witch Bolt did get a good buff in 5.5, there are reasons I'll get into that make it a very worthwhile part of your arsenal here. Also, notably, we have a good 3rd level damage spell option that we can load into a Spell-Storing Item and hand off to our Homunculus, which I now think is a big factor that divides the good Artificer subclasses from the bad ones (Sorry, Alchemists and Cartographers).

Level 3:

Reanimator's Skill Set:

You get two benefits: one is the standard subclass tool proficiency, in this case Alchemist's supplies, or another if you already have it. The bigger part, though, is that you can modify your Spare the Dying cantrip to also create a burst of lightning damage around the target in a 10-foot emanation. Creatures in the area of your choice make a Dex save, taking 2d4 lightning damage on a failure or half as much on a success. The damage goes up by a d4 at levels 5, 11, and 17. You can do this Int times per long rest.

    Effectively, this gives Spare the Dying cantrip scaling, kind of along the lines of Word of Radiance or Sword Burst. Note also that Spare the Dying is now a ranged cantrip by default (getting longer range at each tier). Once again, pay attention to the lightning damage, as this will have additional benefits. It's limited use, but also, how often do you need to stabilize people that also happen to be an enemy. But it might be a good idea to send your pet over to them before you set this off.

Reanimated Companion:

The real headliner, this lets you create an undead construct (undead creature type) as an action using Tinker's Tools (or another set you're proficient with). It takes just an action to do so, and you can do it once for free per long rest, or just spend any spell slot to do it again.

The companion lasts until you finish a long rest or it dies, or you can use an action to dismiss it, or if you die. If it dies, it triggers its Death Burst, which does 2d4 Necrotic damage to creatures in a 10-foot emanation, with a Dex save for half (notably, this will do friendly fire). Like other subclass pets, you need to use your bonus action to direct it to do anything other than dodge, unless you're incapacitated, in which case it can act on its own.

The companion has an AC of 10 plus your Int, HP equal to 5 plus five times your Artificer level (with d8 hit dice equal to your Artificer level) and a speed of 30 feet.

It has resistance to Necrotic and Poison damage, and immunity to Lightning. Indeed, it has lightning absorption, causing it to regain HP for each point of lightning damage it would take.

It also has 60 feet of Blindsight, which is pretty nice for detecting invisible foes.

The companion's attack uses your spell attack modifier for its attack bonus, and does 1d4+ your Int of Necrotic damage on a hit, and prevents the target from making Opportunity Attacks on a hit until the start of its next turn.

    So, the damage here isn't very impressive, but that is often the case with subclass companions. But you can get a bit more damage with it by sending it into swarms of foes and letting it blow up. 2d4 damage isn't amazing if you're only thinking of it as an AoE spell, but it's a decent payoff if your companion is taken out.

    Now, that being said, you have a great way of keeping it alive. I don't actually think it's worth casting Witch Bolt on it, because for the same spell slot you could just make a new one, but when you get Lightning Bolt, catching this in it along with your primary target, that'll give you some easy extra survivability for it.

Level 5:

Strange Modifications:

You can now choose an option to add some modifications to the companion (this is a feature that expands as you level up). Each time you create a companion, you get to pick one of the following:

Arcane Conduit lets you cast spells as if you were in the Companion's space (using your own senses). Additionally, once per turn, when you cast an Artificer spell from the Evocation or Necromancy schools, you can add your Intelligence modifier to one damage roll of the spell.

    So, this I suppose is the rough analogue for something like Arcane Firearm. I'd say this is probably going to be a default option. Spells that will benefit from the damage bump should be Witch Bolt, Lightning Bolt, Blight, Acid Splash, Fire Bolt, Poison Spray, Ray of Frost, Shocking Grasp, and Thunderclap.

    In fact, I actually think there's an argument to be made for making your primary damage cantrip Shocking Grasp, because you can still stay at the back and just use it via your companion, and then if you need to heal them up, just shock 'em a bunch of times.

Ferocity changes the damage die for the companion's attack from a d4 to a d6.

    Very little to say here. This winds up being just 1 damage higher, and I don't think you get nearly as much of a benefit as Arcane Conduit. (Sad - in the UA, you could give your companion two attacks, which would have actually been some really nice damage scaling).

Level 9:

Improved Reanimation:

The damage of the companion's Death Burst goes up to 4d4, and the necrotic damage dealt by your companion ignores resistance.

    Necrotic resistance is very rare, though I suspect you might encounter it more in a Ravenloft campaign. The boost to Death Burst is cool, but it's still not enormous damage (though you'll have more spell slots to burn on disposable companions at this level).

Macabre Modifications:

You gain new options for your Strange Modifications, and furthermore, you can now pick two each time you create an companion.

Bloated make the companion Large, and when it hits a Large or smaller creature, they get pushed back 10 feet. Additionally, you can add your Int modifier to its Death Burst damage.

    This is a pretty solid option. The size and the pushing both help this do some battlefield control, and the Death Burst damage now becomes 14 or 15, with a large radius (given that an emanation is bigger from a larger creature). I'd still probably always use Arcane Conduit, but this is a solid choice for your second modification.

Gaunt increases the companion's speed to 45 feet, and it gets the same as a climb speed, effectively with Spider Climb. Creatures of your choice that start within 10 feet of the companion have to make a Wisdom save or become frightened until the start of its next turn.

    And honestly, this is really good too. The speed is nice to have, and the fear is also potentially really good - frightening a whole bunch of monsters that are near the companion.

Moist gives the companion a swim speed equal to its speed and allows it to squeeze through spaces as narrow as 1 inch. Also, when the companion takes damage from a creature within 10 feet of it, the attacker takes acid damage equal to your Int modifier.

    Another really solid choice - obviously the clear winner if you find yourself in aquatic combat, but the backlash damage is also pretty great - especially if the monster hitting it has multiattack. At worst, this disincentivizes foes from attacking your pet.

Level 15:

Refined Reanimation:

You get the following:

Facilitated Revival lets you cast Raise Dead once per long rest without expending a spell slot or material components once per long rest, as long as you have Tinker's Tools or another type of Artisan's Tools to use as a focus.

    This is pretty nice: while Raise Dead is the most onerous resurrection spell, you're also hopefully not going to have to cast it more than once a day, so this can be a little insurance for the party.

Life Transfer lets you use a reaction when you or your companion take damage. You gain HP equal to your companion's current HP and then the companion drops to 0, triggering its death burst.

    I'd like some clarification on whether you take the value of their HP from before or after the hit - I'm assuming after. Still, think about it this way: if you're out of combat, and you want to heal up, send your companion fifteen feet away from anyone you don't want to hurt, then make a ranged attack at it with, like, a sling or a something else that does minimal damage, and then blow it apart to heal yourself for, at this level, 80 HP minus the damage you did. And then get a new companion for a 1st level spell slot. I'm... I'm not sure that WotC thought of that. That is an insane self-heal.

Superior Modifications, then, lets you choose three options for Strange Modifications, up from two (but with no new options).

    And hey, this means you could make an Arcane Conduit Bloated but somehow also Gaunt companion.

Overall Thoughts:

    In terms of pure power, I don't think this is going to be a damage power-house, at least not any more than an Armorer and probably falling behind a Battle Smith.

    But I love the design, because this is so customizable. The fact that you can make companions as an action means that you'll very easily be able to pick whatever suits your current purposes best.

    While Death Burst is probably going to wind up being more of a fun bonus than a major source of damage (though it starts to get significant with a Bloated companion) it's delightfully grisly and thematic.

    I've generally wanted to play an Armorer, and also somewhat a Battle Smith, but I think if I was doing a Ravenloft campaign, I'd strongly consider this subclass, and I think it could be a lot of fun.