In a dangerous surgery, you remove your stomach and replace it with a monstrous version from a creature such as a plague spewer, plague walker, or famine spirit. It generates a revolting bile that you can vomit at your foes.  It bulges against your skin. 

Whether you have to make Medicine checks to perform the surgery and whether you had to go on a quest to acquire the stomach, 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.  Any spells that you cast through this necrograft require no verbal, somatic, or material components, even expensive ones. 

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.

Stage 1

You can Vomit Bile as an action.  You can use Vomit Bile in two different ways.  One acts as the Acid Splash cantrip, except the range is only 15 feet and it deals additional damage equal to your Constitution modifier.

The second way acts as the Grease spell, except that all the affected squares must be within 15 feet of you and any creatures that fail their saving throw take acid damage equal to your Constitution modifier.

You can spend two uses of Vomit Bile to use it as a bonus action.  However, you cannot use Vomit Bile more than once a round.

You can Vomit Bile three times between rests. When you finish a short or long rest, you regain your expended uses.

You can spend an action to belch forth noxious gases from your gut.  You gain the Belch ability.  This acts as Fog Cloud, but it requires no concentration, the area of effect is adjacent to you and is green in color, and the duration is 10 minutes.  You must finish a long rest before you can use this feature again.  You cannot use Belch and Vomit Bile in the same round.

Stage 2

When you use Vomit Bile to reproduce Acid Splash, affected creatures now take half damage on a successful saving throw instead of no damage.

When Vomit Bile is used to reproduce Grease, any creatures that fail their saving throw take 1d4 plus your Constitution modifier acid damage.

You can use Vomit Bile four times between rests.

When you use Belch, you can make the cloud poisonous. Each creature that is within the cloud at the start of its turn must make a Constitution saving throw against poison. On a failed save, the creature is poisoned until the start of its next turn. Creatures that don’t need to breathe automatically succeed on this saving throw.  This version of Belch lasts 1 minute.  You are immune to this gas. 

Stage 3

When you use Vomit Bile to reproduce Acid Splash, creatures that fail their saving throw have their speed reduced by 10 feet until the end of their next turn. 

If you use Vomit Bile to reproduce Grease, any creatures that fail their saving throw take 2d4 plus your Constitution modifier acid damage.

You can use Vomit Bile five times between rests.

When you use Belch, you can make the cloud even more powerful.  It acts like Stinking Cloud except that it requires no concentration and the area of effect is adjacent to you.  You are immune to it.

Stage 4

You can use your action to Vomit Everything, unleashing a devastating breath weapon in a 15-foot cone.  Creatures within the area must make a Dexterity saving throw.  Creatures that fail take 12d6 acid damage  and have their speed halved as they are covered in thick vomit.  Creatures that succeed take half damage and are not covered in vomit.  Creatures covered in vomit can spend an action to attempt to free themselves from the viscous bile.  If they succeed on a Strength saving throw, the effect ends.  After you use this feature, you can’t use it again until you complete a long rest. You can’t use Vomit Everything and Vomit Bile in the same turn.

If you use Vomit Bile to reproduce Grease, any creature that fail their saving throw take 3d4 plus your Constitution modifier acid damage.

If you use Vomit Bile to reproduce Acid Splash, it deals d8s damage instead of d6s.

When you use Belch, you can create a lethal cloud of gas.  This acts as Cloudkill, except that it requires no concentration and the area of effect is adjacent to you.  You are immune to it. 

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.