Uumuu

Scyphozoan Membrane: Uumuu is immune to all sources of damage while the membrane is active. The membrane has 100 Hit Points, an AC of 20, and it is immune to all damage except Piercing. In addition, it gains the following traits:

 Regeneration. The Membrane regains all of its HP at the end of each round as long as Uumuu is alive, and removes all status conditions currently afflicting it. If the Membrane dies, this trait does not function for 2 turns after its death.

Bouncy Exumbrella. When a creature targets Uumuu with a Melee attack while the Membrane is active, they must succeed on a DC 19 Strength saving throw or be pushed up to 10 feet away from Uumuu in a straight line.

Explosion Sensitivity. If either the Membrane or Uumuu itself is hit by the exploding core of an Ooma, it instantly dies.

Static Chase. Electrical energy surrounds one target within 30 ft. At the end of the targeted creature's next round, electricity fills a 5 ft cube where they were when targeted, forcing all creatures within the cube to make a DC 20 Dexterity saving throw or take 10d6 lightning damage, or half as much damage on a successful save. This effect repeats for 3 rounds or until Uumuu's membrane is inactive.

Lightning Burst. Ranged Spell Attack: +13 to hit, range 30 ft, up to 10 creatures, 5 ft cubes, or objects within range. Electricity momentarily blossoms in all targeted spaces, and all creatures within 5 feet of a target must make a Dexterity saving throw or take 8d6 lightning damage per source, or half as much damage on a successful save.

Electrified Membrane. When a creature touches Uumuu while the Membrane is active, Uumuu can use its reaction to deal a number of D6s of lightning damage to that creature, equal to Uumuu's remaining HP divided by 10.

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

Summon Ooma (Costs 2 Actions).  Uumuu summons 2 Oomas under its control that take their turns directly after Uumuu's. If Uumuu is hit by the Death Burst of these Uomas, Uumuu's membrane deactivates for 1 round. 

Move (Costs 1 Action) Uumuu moves up to its movement speed.

Big Buumuu (Costs 3 Actions) Uumuu instantly kills all Oomas that were summoned by its Summon Ooma legendary action. If  throw for this round.