So, your player has been reading Altered Carbon or playing some Deus Ex and wants their sorcerer to have an awesome cybernetic arm attached that can do all kinds of cool stuff... No problem!
There are a few different options you could choose from to make this happen. You could go the Eberron route and give them something like the Prosthetic Limb but that requires an attunement slot—this is fixed by Tasha's—and it does not actually do anything different from a standard limb. You could use something like the Armblade or the Arcane Propulsion Arm both of which are cool but require you to own a copy of Eberron if you want to remember how they work (which I do encourage, it is a great book and I would kill to meet Kieth Baker) or sail the high seas of the internet to find the stats (which I take a morally neutral stance on in general).
Alternately, you could use the rules below to really give your players that "going under the knife and coming up with a cool new ability" feeling.
Grafted Magic Items
Grafts are magical items implanted into the bodies of their wielders. The process of grafting is dangerous but for those willing to take the risk, the rewards are great...
Unless otherwise stated in the item's stat block, a grafted item must be implanted into a character with the help of a Non-Player Character who specializes in grafting or a Player Character proficient in the Medicine skill (Grafter).
The Grafter must succeed on a Wisdom (Medicine) check the DC of which is determined by the rarity of the item as shown in the table below.
On a failure, the Grafted Item's "Improperly Implanted" feature is triggered as outlined in the item's description.
Example Grafted Items
Blazing Eye
Grafted Item, Common, No Attunement Required
This magical device replaces a lost eye. As an action, you cause a beam of power to shoot from this eye at target that you can see within 60 feet. The target must succeed on a Dexterity Saving Throw equal to 8 + your Wisdom Modifier or take 1d6 force damage.
Improperly Implanted. If the Blazing Eye was improperly implanted, you cannot benefit from Darkvision. This can be corrected with the use of the spell Lesser Restoration or similar magic.
War Limb
Grafted Item, Rare, Requires Attunement
This magical device replaces a lost arm. While attuned to a War Limb, your Strength or Dexterity Score increases by 1.
Additionally, over the course of a short rest you can activate one of the arms enchantments granting it one of the following abilities. Only one ability can be active at a time.
Concussive Bolt. As an action, you make a ranged attack against a target that you can see within 60 feet. On a hit, that creature takes 1d10 Thunder damage.
Medical Appendage. As an action, you can touch a living creature that has 0 hit points and infuse them with life force. The creature gains one hit point. This action has no effect on the undead.
Clawed. You can roll a d8 in place of the normal damage of an unarmed strike and your attacks deal slashing instead of bludgeoning damage.
Improperly Implanted. If the War Limb was improperly implanted, you must roll 1d6 every time you attempt to change the arm's enchantment. On a 1, the enchantment sticks and you cannot change it again. This can be corrected with the use of the spell Greater Restoration or similar magic.
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