Ritual Summoned: The Abyss Elemental is summoned over a period of one month using four onyx-cast arcane focus instruments. For each instrument destroyed before the elemental can breach through to the material plane, its Hit Point modifier is reduced by 30. In addition, each instrument destroyed reduces the elemental's CR by 1, down to a minimum of 12.

If the Abyss Elemental is reduced to 0 hit points, it returns to its home plane.

Multiattack: The elemental makes two claw attacks or one breath attack.

Claw: Melee attack; +8 to hit, reach 10 feet, one target. Hit: 16 (3d8 + 3) slashing damage and 16 (3d8 + 3) necrotic damage.

Shadow Breath (Recharge 5-6): The elemental exudes a black smoke in a 30 foot cone in front of it. Each creature in range must make a DC 17 Constitution saving throw, taking 44 (8d10) necrotic damage on a failure and half as much on a success.

Sentinel: The Elemental can take attacks of opportunity even if the creature used the Disengage action. Additionally, if the attack is successful, the creature's speed is reduced to 0 until the end if its turn.