I have really enjoyed the Netflix Castlevania series and have found the Forgemasters (Issac and Hector) to be very interesting characters, especially in the way they construct armies of fiends and undead. Unlike a conventional necromancer, their creation of these creatures seems more tied to the use of tools and careful magical crafting. For this reason, I felt inspired to write a necromancy-esque Artificer subclass drawing inspiration from the series. To compare to a necromancer, the overall size of an army they can create in a short period of time is functionally smaller but more robust by making use of both Animate Dead and some of the new summoning spells from TCoE. 

Forgemaster

Forgermaster artificers are craftsmen who work in raising creatures from the dead and constructing hellish servants. Given the nature of their work, these artificers are usually drawn into the service of Liches, Vampire Lords, and other powerful necromancers who can provide them the materials they need for their work

 

Forgemaster Spells

Starting at 3rd level, you always have certain spells prepared after you reach certain levels in this class. These spells count as Artificer spells for you but do not count against your number of spells (or cantrips) known.

 

Artificer Level	Spells
3rd	Chill Touch, Inflict Wounds, Hellish Rebuke
5th	Prayer of Healing, Summon Beast**
9th	Animate Dead, Summon Undead**
14th	Blight, Shadow of Moil*
17th	Negative Energy Flood*, Mass Cure Wounds

 

*Spell found in Xanathar’s Guide to Everything

**Spell found in Tasha’s Cauldron of Everything

 

Tools of the Grave

When you choose this specialization at 3rd level, you are able to empower an object with necrotic energies for forging the souls of the forgotten. When you finish a long rest, you can select either a set of tools which you are proficient in or one item bearing one of your infusions to empower this way, with the object gaining the following properties: 

• When the object is used as a spellcasting focus for casting a spell which deals necrotic damage, you can treat values of 1 rolled on the damage dice as 2 instead.
• When casting a spell as a ritual, the time needed to cast the spell is reduced by half provided you are holding your tools of the grave.
• If the object is a weapon, once per turn when you hit a creature with an attack using your tool of the grave you can deal additional necrotic damage equal to your Intelligence modifier. 

You can only have one object acting as your tools of the grave at a time. When you empower a new object as part of a long rest, the previous object immediately loses the benefits of this feature. 

Crafted Summoning

Starting at 5th level, while within 5 feet of a body which has been dead for no more than 24 hours and while holding your tools of the grave, you can cast a spell from the necromancy or conjuration spell list which has the effect of summoning one or more creatures, using the body as an additional component for the spell. A creature summoned this way is treated as an undead or fiend (your choice) in place of the type listed in its stat block. It also gains temporary hit points equal to three times your proficiency bonus. 

 

Life unto Death

Starting at 9th level, once per turn when you deal necrotic damage to a creature you can choose to grant temporary hit points equal to twice your Intelligence modifier to yourself or an undead or fiend within 10 feet of you. You can use this feature a number of times equal to your Intelligence modifier and regain all uses following a long rest. If the effect dealing necrotic damage targets more than one creature, the healing received is derived from only one creature hit by it. 

Additionally, when you use a spell or effect which restores hit points you can ignore any part of the spell or effect which prevents it from targeting undead. 

 

Army of Darkness

At 15th level, after using your Crafted Summoning feature you can choose to bind the summoned creature permanently to your current plane of existence under your service. When you do so, the spell’s duration is extended to last until the creature is killed or you are killed and does not require concentration. Once you bind a creature this way, you cannot do so again for 24 hours. 

Any undead or fiends under your service due to your Artificer spells can be commanded as part of the same bonus action on your turn.

Forgemaster Spells shouldn't have three spells listed at 3rd level, but just two. 

You could state the extra cantrip like it's done in some cases (see below):

Bonus Cantrips
At 1st level, you learn the sacred flame and light cantrips. They count as warlock cantrips for you, but they don't count against your number of cantrips known.

Yours could be as well look like this:

Bonus Cantrips
At 3rd level, you learn the chill touch cantrip. It counts as an artificer cantrip for you, but doesn't count against your number of cantrips known.

Other than that, I'm not sure on what else you could do to improve or change this-

Something important to note/realize about this subclass level 15 feature, undead created through most spells don't have their control linked to the spell's duration itself but rather the spell is instantaneous and you have to recast the spell every 24 hours to retain control over the undead creature so as written it wouldn't actually permanently bind many if any undead to the bearer of this class's user, at the very least animate undead wouldn't. If this is intended to work on them you may want to find a way to word it to include permanently binding the the permanently created undead to your will as well. Otherwise pretty decent job with this subclass, it feels like it gets to the theme/flavor of a castlevania forgemaster pretty well and doesn't really do it in an unbalanced fashion. By 15th level the permanent summons you're able to add to your army/retinue once a long rest is at slow enough rate that it shouldn't be too problematic balance wise, not much more than most things by that level are anyways.

Think that like most minionmancers this may  require discussion over how much and how strong you can have tagging with your party at a given time but should be workable I reckon