Anyparxia, the Dimensional Gatebreaker

Void Step. All attacks of opportunity against this creature are made at disadvantage.

Spacious Strike. An attack from this creature at a distance almost seems to erase space between itself and its target. Melee attacks from this creature can be made at 10 feet instead of 5.

Multiattack. This creature can make two claw attacks on its turn.

Claw. Melee Weapon Attack: +3 to hit, reach 5-10 ft., 1 target. Hit: 15 (3d8 + 8) slashing damage. 

The anyparxia can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The anyparxia regains any spent legendary actions at the start of its turn.

Parting Claw (Costs 1 Action). The wound from Anyparxia's attack leaves a feeling of muscles expanding and contracting, almost as if they are folding upon each other. The target must make a DC13 Constitution saving throw. On a failed save, roll a d12 and subtract the result from the target's maximum hit points. The target regains hit points lost this way after a long rest.

Vacuous Gaze (Costs 2 Actions). Locking eyes with this creature sends the target through waves of agony and despair, almost breaking their spirit. The target must make a DC13 Wisdom saving throw. On a failed save, the target is incapacitated and their movement speed drops to 0. An ally can use the Help action to shake the creature out of their dazed stupor. Moreover, the creature can make a DC13 Wisdom saving throw at the beginning of their turn. A creature that successfully saves is immune to Anyparxia's gaze for the next 24 hours.

Assimilate (Costs 3 Actions). The Anyparxia proceeds to overwhelm and control its target's body, becoming a shadowed form upon completion. The target must make a DC 13 Constitution saving throw when this action is used and at the end of their next turn. An incapacitated or unconscious creature automatically fails. On a failed save, the target gains 3 levels of exhaustion. On a second failed save, the target gains 3 more levels of exhaustion, ending their life and becoming an Entity Between. On a successful save, the target resists the merging and loses any exhaustion it has gained. A target that successfully saves is immune to this ability for the next 24 hours.