Artifact, legendary (requires attunement by a caster).
The artifact is sentient: INT 20, WIS 12, CHA 10.
Alignment: Lawful Neutral.
It can communicate only telepathically and only in the Abyssal tongue. The artifact is blind and deaf, but recognizes the genetic imprint of its creator.
Properties:
The Prosthetic Arm can be used as a spellcasting focus.
Mind and Blades Synchronized: When you cast a spell, you can use a bonus action to make a weapon attack. If you do so, add half the damage dealt by the previous spell, converted to force damage.
Reactive Interface: When you make an attack with a weapon wielded with your arm, you can choose to channel a 1st-, 2nd-, or 3rd-level spell already cast this turn to add the spell's effect. The spell's effects last half as long, and the target has advantage on saving throws against the spell's effects.
Breach in the Depths: Once per day, you can open a portal to the Far Realm, causing the area to become covered in tentacles for 300 feet, becoming difficult terrain with the “Evard's Black Tentacles” effect. Any creatures in the area affected by the entangled condition will be pulled 1d6 meters toward the portal each turn until they are swallowed up unless they break free with a DC 15 Strength check. Additionally, the entire area is affected by Far Realm as long as the portal remains open. 1d4 random hostile aberrations may appear each turn. Casting Time: 2 rounds. Duration: 1 hour. The user takes 1d10 lightning damage.
Secret Function
Portal to the Keliphot: Using the blood of Erakos or his clone and the password "Rex Profunditatum," the artifact opens a portal to the 666th layer of the Abyss. The portal remains open for 1 hour. Casting time: 1 minute. Cooldown: 1 day. The user takes 12d6 points of lightning damage.
The arm dislikes being touched by anyone who does not share its creator's genetic signature. The arm "loves" its creator. Therefore, any user other than Erakos or his clone must make a DC 12 Wisdom saving throw each time they use it or suffer a panic attack (fearful) lasting 1d8 hours. Additionally, they roll a d6, which determines one of the following:
1- the effects of the spell "Crown of Madness."
2- the effects of the spell "Feeblemind."
3- the effects of the "Confusion" spell.
4- sleep effect for 1d10 minutes.
5- 1d20 psychic damage.
6- -1d4 to saving throws, attack rolls, and damage rolls.
To attune to it, the user must make a Charisma saving throw of 20. If the saving throw fails, the user will be "lobotomized," taking 4d20 psychic damage + 1d10 lightning strikes and permanently reducing their Intelligence to 2. If the character is reduced to 0 HP, his/her head explode.
Notes: There is no artifact like this in the entire Multiverse. Èrakos forged it in the titanic forge of the 666th layer of the abyss, using knowledge gleaned from wandering the Far Realm. It is not just a prosthetic; it is far superior to a physical arm. The craftsmanship is marvelous: the delicate lines of the perforated and chiselled adamantium shell envelop bundles of tendons and muscles made of metallic fiber. The gold and silver inlays depict surreal and metaphysical scenes in which ancient aberrant creatures compete over lost universes. The first prototype was created by Erakos in the forges of Pazunia, after his original arm was torn off and devoured by demons. It was thanks to that prototype that Erakos managed to escape from slavery., The user must make a Charisma saving throw of 20. If the saving throw fails, the user will be "lobotomized," taking 4d20 psychic damage + 1d10 lightning strikes and permanently reducing their Intelligence to 2. If the character is reduced to 0 HP, his/her head explode.