In a very lengthy and agonizing operation, you graft onto yourself a significant portion of a slain zombie’s muscles. Your weight increases by 50%.

Whether you have to make Medicine checks to perform the surgery and whether you had to go on a quest to acquire the zombie, are details best left up to your GM.

You can expend a spell slot to activate the necrograft.  The more powerful the spell slot is, the more powerful the necrograft is.  When you expend a spell slot, the necrograft is active for 24 hours or until you finish a long rest, whichever comes first.  If you are a warlock, any spell slot that you expend to cast this spell is not regained until you finish a long rest.

The necrograft has four different power levels: Stage 1, Stage 2, Stage 3, and the most powerful, Stage 4.  The benefits are cumulative, if you activate the necrograft at Stage 2, you also gain the benefits described in Stage 1.  The DC of any features gained through this necrograft is your spell DC.

Activating the necrograft at Stage 1 requires expending a spell slot of at least 3rd level.

Activating the necrograft at Stage 2 requires expending a spell slot of at least 5th level.

Activating the necrograft at Stage 3 requires expending a spell slot of at least 7th level. 

Activating the necrograft at Stage 4 requires expending a 9th level spell slot.

At any given time, you can have up to four necrografts active.  One cannot exceed Stage 1, one can be up to Stage 2, one can be up to Stage 3, and one can be up to Stage 4.

This spell was originally designed as part of the Necrografter class, which can be found on DnDBeyond. It is intended to be used with the spell Unhallowed Arts, which can also be found on DnDBeyond.

Stage 1

You gain the Powerful Build ability.  You count as one size larger when determining your carrying capacity and the weight you can push, drag, or lift.

You gain the Invoke Abomination ability.  You can spend a bonus action to swell in size, becoming a looming monstrosity.  This acts as the Enlarge spell and has the same duration, but requires no concentration.  However, only your unarmed strikes (including Bone Spikes) deal the additional 1d4 damage.  Once you use this feature, you can’t use it again until you finish a short or a long rest. 

You can reroll a Strength saving throw or ability check that you fail.  If you do so, you must use the new roll, and you can't use this feature again until you finish a long rest.

Stage 2

When you hit a creature with an unarmed strike on your turn, you can use a bonus action to attempt to grapple the target.

You can make a special bite attack that rattles the enemy’s psyche. You gain the Brain Gnaw ability.  When you are grappling a creature and hit it with an unarmed strike, it takes additional piercing damage equal to your character level and must make a Constitution saving throw.  If the target fails, the first time in the next minute when it makes an Intelligence, Wisdom, or Charisma saving throw, it must roll a d4 and subtract the number rolled from the saving throw.

Creatures without a head or brain are immune to this ability.  Once you use this feature, you can’t use it again until you finish a short or long rest.

You can use Invoke Abomination for up to 10 minutes between rests.  You use it in increments of 1 minute.

Stage 3

Zombies keep on coming. You gain the Relentless ability. You can reroll a saving throw against being paralyzed or stunned that you fail. If you do so, you must use the new roll, and you can't use this feature again until you finish a short or long rest.

You are exceptionally good at smashing things.  You gain the Door-Breaker ability.  Your unarmed melee attacks against objects, vehicles, and structures deal an additional 50% damage.  Add 5 to any Strength checks you make to destroy things. 

You can use Invoke Abomination for up to 1 hour between rests.

Stage 4

You can use Invoke Abomination at will.  It has an unlimited duration.

If you have cast the spell Unhallowed Arts (see DnDBeyond), your unarmed strike base damage increases to 1d10.

When you are using Invoke Abomination, you can make a nigh-unstoppable charge.  You gain the Zombie Rampage ability.  You can spend an action to move up to twice your speed in a straight line.  You can move through creatures.  A creature size Huge or smaller that you move through must make a Dexterity saving throw or take 6d10 + 10 bludgeoning damage on a failed save or half on a success as you barrel straight into it.  If you have the Elemental Cadaver necrograft, you can use Elemental Surge to increase this damage.  Creatures that fail their save are also knocked prone.

While moving, you can also make unarmed attacks against any number of creatures within range, although you can attack a given creature only once and cannot attack a creature that you have moved through.  Any creature that you hit must succeed at a Strength saving throw or be knocked prone and moved up to 15 feet in any direction of your choice.  You can choose to move a creature into the path of your charge, forcing it to then make a Dexterity saving throw or take damage as described above. 

If a Gargantuan creature is in your path, your charge ends, but make an attack against it with advantage. If it hits, the target suffers additional damage equal to twice your necrografter level.

If an object, vehicle, or structure blocks your path, it similarly takes 6d10 +10 bludgeoning damage, which is increased by Door-Breaker.  If you destroy it, you can continue your charge.  If not, your charge ends.

Once you use this feature, you can’t use it again until you finish a long rest.

Other necrografts can be found on DnDBeyond.  Here is the complete list:

Abomination Gut: You can vomit acid and belch poison.

Bonemail: You can better protect yourself and your allies.

Bone Spikes: You gain long-range attacks and can retaliate against foes.

Elemental Cadaver: You can channel fire, frost, or lightning.

Ghoul Claws: You can gouge and paralyze your foes as well as swim through the ground.

Mummified Eye: You gain a fearsome gaze, bonuses to initiative, and access to some divination spells. 

Lich’s Skull: You heighten your mental defenses and gain minor spellcasting plus knowledge.

Paladin’s Tomb: You’re the opposite of a holy man, but you can fight like one.

Two Corpses in One Grave: Sharing your body with another soul, you can change your appearance and shock your opponents with a hideous metamorphosis.

Wight Heart: Your attacks deal extra damage and can weaken your enemies.

Vampiric Grace: You can move fast, crawl on walls, and drink the blood of your foes.

Vampiric Soul: You can bend the minds of others to your will and call creatures of the night to serve your bidding.

Zombie Hulk: You can increase your size and use special melee attacks.