Phase Switch. Every turn, at the beginning of Supremus's turn, it will change phase. It starts the combat in phase 2 without any other ability/action activated (i.e. Fixate). Phases will change the ability that Supremus and his lair do.

When switching into phase 1, Supremus will reset all the damage dealt counters.
When switching into phase 2, Supremus will do the Fixate bonus action.

Legendary Resistance (2/Day). If Supremus fails a saving throw, it can choose to succeed instead.

Ready to Fight. Supremus has advantage on initiative rolls.

Uncheangeable size. Supremus is immune to any spell or effect that will change it's size.

Multiattack. Supremus makes one attacks by the lists below depending by the phase. Then do one Attack.

Attack. Melee Weapon Attack: +9 to hit, one creature. Hit: Supremus moving itself less possibile, attacks the nearest target that has dealt the highest damage with both his fists doing 16 (2d12+4) bludgeoning damage.

Phase 1

Hateful Strike. Melee Weapon Attack: +9 to hit, one creature. Hit: Supremus uses one of his fists smashing it to the ground hitting one target in melee range (tends to exclude the nearest one that has dealt the highest damage, if there is olny one target, it will hit that one) doing 24 (5d8+4) bludgeoning damage.

Phase 2

Molten Punch. Melee Weapon Attack: one creature, 10ft reach. Hit: Supremus extends one of his punch upon reaching his objective, the target must make a DC 20 Dexterity saving throw, taking 52 (8d12+4) bludgeoning damage and being knocked back by 30ft falling prone on a failed save, or evading completely the attack on a successful one. If the target dies, Supremus will execute immediatly another Fixate action.

 Fixate. Supremus will choose a random target. He will  do a bonus action Disengage from any enemies that he has against it and rush towards the designed target doing a dash as bonus action. Every creature that will be hitted by Supremus must do a DC 16 Dexterity saving throw to evade the charge, taking 10 (2d10) bludgeoning damage and falling prone on a failed saving throw, pushed back by 10ft in the direction of the attacker on a successful one. All the attacks done by Supremus will be done to that target if able. Otherwise Supremus will charge that target doing another dash bonus action.

Total 2 actions per turn.

Attack (Costs #1 Action). Melee Weapon Attack: +9 to hit, one creature. Hit: Supremus attacks the nearest target with both his fists doing 16 (2d12+4) bludgeoning damage.