Chrona, the First Overlord

Fey Alien. When Chrona is targeted by an effect that targets a humanoid, she is considered an Aberration or Fey. Any effect that detects an Aberration or Fey detects her.

Objects In Motion. When Chrona takes Fire, Force, Lightning or Radiant damage, she gains temporary hit points equal to 5 times the damage she would have taken.

Stay In Motion. When Chrona gains temporary hit points, they are added onto any temporary hit points she has.

Multiattack. Chrona makes ten attacks with any weapon she has or her Vedanium Gauntlets. If a spell that deals Fire, Force, Lightning or Radiant damage is affecting her, she makes an additional amount of attacks equal to the spell level.

Vedanium Gauntlets. Melee Weapon Attack: +14 to hit, reach 5 ft., 1 target. Hit: 1d10 + 17 bludgeoning damage. 

Vedanium Entropy (1/day). Chrona uses this as a last resort. Chrona takes 30 points of radiant damage that overcomes her immunity and Objects In Motion feature, after which she gains 400 temporary hit points. These temporary hit points degrade rapidly, at Initiative count 20 she loses 80 temporary hit points and once all of her temporary hit points are gone she falls to the ground as she suffers 5 points of exhaustion and cries in pain as her skin and insides are getting melted from the atomic fission.

Kinetic Accretion (Recharge 5-6). As a reaction to taking damage, Chrona becomes immune to all damage for that instance of damage. After the instance is resolved, she gains temporary hit points equal to half the damage taken.

Chrona 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. Chrona regains spent legendary actions at the start of its turn.

Attack. Chrona makes one attack with her Vedanium Gauntlets

Aberrant Fey Presence (Costs 2 Actions). Every creature within Chrona's line of sight must make a Wisdom saving throw (DC 21) or be Charmed and Frightened by Chrona.

Atomic Release (Costs 3 Actions). Chrona loses X temporary hit points and forces all creatures within a 30 foot sphere to make a Constitution saving throw or suffer an amount of Radiant damage equal to half of X (to a maximum of 100)