Bonemail (Necrograft)

In a lengthy and painful operation, you graft protective bone plating over practically all of your body.  The bones may have originated from almost any sort of creature. 

Whether you have to make Medicine checks to perform the surgery and whether you had to go on a quest to acquire the bones 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. 

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

Your gain a +1 bonus to AC as long as you are not wearing heavy armor.

As a reaction when you suffer a critical hit, you can turn that attack into a normal hit. Any effects triggered by a critical hit are canceled. Once you use this feature, you must finish a long rest before you can use it again.

Additionally, you get the Bone Shield ability, allowing you to temporarily extend your plating to protect a nearby ally. When a creature you can see attacks a target other than you that is within 5 feet of you, you can use your reaction to impose disadvantage on the attack roll.  Once you use this feature, you must finish a short or long rest before you can use it again.

Stage 2

You can use Bone Shield three times between rests. Additionally, you can now use it to protect a target within 10 feet of you.

You can use Bone Shield in a limited offensive capacity.  If you take the Attack action on your turn, you may after attacking spend a use of Bone Shield as a bonus action to try to shove an opponent within 10 feet of you. 

Stage 3

You can now use Bone Shield to protect an ally within 15 feet of you. Additionally, when you use Bone Shield, instead of imposing disadvantage on an attack, you can give an ally within 15 feet of you advantage on a Dexterity saving throw against an effect for half damage.  If the target succeeds, they take no damage.

You can use your iron-hard bones to pummel your foes.  When you hit with an unarmed melee strike that deals bludgeoning damage, you can make a Marrow Whack.  The target takes additional damage equal to your character level and the next attack against it before the end of your next turn has advantage as it reels from the impact. Once you use this feature, you must finish a long rest before you can use it again.

You can attempt to shove enemies within 15 feet of you. When you successfully shove an opponent with Bone Shield, it takes damage as if hit by one of your unarmed strikes. 

Stage 4

When you use Bone Shield to impose disadvantage on an attack roll, the creature making the attack has disadvantage on not only that attack, but subsequent attacks on the same target for the rest of its turn.

When you extend your Bone Shield to give advantage on a Dexterity saving throw, the target takes half damage even if they fail their saving throw.

You gain an additional special use of your ability to extend your bone plating, Immortal Bone Shield.  You can protect yourself and all allies within 15 feet of you as a reaction.  Until the beginning of your next turn, you and all allies within range have advantage on Dexterity saving throws and all attack rolls against you and your allies within range have disadvantage.  If you or your allies succeed on a Dexterity saving throw while affected, no damage is taken. If they fail, they take half damage anyway.  Allies that move more than 15 feet away from you lose the protection given by this ability.

Once you use this feature, you must finish a long rest before you can use it again.

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.