Annihilydra

Hold Breath. Can hold its breath indefinitely.

Reactive Heads. Can make 1 action for every head it has, but can only move its body once per turn. It has 5 heads. 

Omnisense. Knows the location of any living creature within 150 feet of each head. 

Neck Extension. Can extend the length of each neck to 300 feet or less.

Bite. Bites at an enemy. +10 to hit, reach 10 ft. One target. Hit: (1d8+10) piercing damage. 

Bypass beam. The jaws of one head open as it shoots a beam of anti-matter at an opponent. 200 feet long, 10 feet wide. Each creature in that line must make DC 20 Dexterity saving throw. On failure, they take (1d10+5) force damage and (1d10+5) lightning damage. On a success they take half as much damage. 

Stomp. Annihilydra rears up on its hind legs and then stomps back down, causing an earthquake and shards of rock to jolt out of the ground. Everyone within 80 feet starting at the center of the Annihilydra in every direction on the ground must make DC 20 Dexterity saving throw. On a failure, they take (3d8+5) bludgeoning damage and are stunned for a turn. On a success they take half damage and are not stunned.  

Gravity Shift. Annihilydra increases, decreases, or changes the direction of gravity on the field. Lasts for 1 round. If gravity is decreased, everyone floats up 40 feet. If gravity is increased, everyone gets stuck to the ground and cannot move by normal means. If the direction of gravity is altered, everyone is thrown to the specific direction until they hit a wall meaning for 1 round gravity now works in that direction, and they must make DC 15 Dexterity saving throw. On a failure, they take (1d8+5) bludgeoning damage. On a success they take half damage. This action can only be used once per turn. 

Constrict. One of its necks extend to an opponent and wrap around them, constricting and suffocating them. +15 to hit, reach 150 ft. One target. Hit: (1d20+10) force damage and are stunned for 1 turn. 

Doom Roar. One head roars loudly. Annihilydra does double damage for its next attack. 

No Bonus Actions.

Opportunity Dodge. When an opponent makes an avoidable attack at one of the heads, attempt to dodge. Before an enemy rolls for damage, the Annihilydra rolls 1d20. If that roll was a Nat 20, it takes half damage. 

Opportunity Attack. When a hostile entity moves away from you, attempt to bite at them. 

Annihilydra can only take 3 legendary actions. Only one legendary action can be used at a time and per turn. If one of these is used, Annihilydra can not make any other actions or reactions that turn. 

Astral Beam. All 5 heads are needed, as they all shoot their beams together, combining them into a single beam of absolute destruction. 1000 feet long, 40 feet wide. Each creature in that line must make DC 25 Dexterity saving throw. On failure, they take (1d20+40) force damage and (1d20+20) lightning damage. On a success they take half as much damage. After this, there is an explosion as the beam makes contact with the ground, thrusting shards of rock everywhere. All opponents take (1d20+10) bludgeoning damage. 

Starry Nights. The ceiling above Annihilydra disintegrates, as energy from all the visible stars in the night sky is pulled down, causing a shower of hot ray star dust to fall on the field. All opponents in the same room take (4d20+10) fire damage. 

Rewind. This legendary action also counts as a reaction. The eyes on the heads of Annihilydra glow a bright white as time itself is turned back to the start of Annihilydra‘s previous turn. If this doesn’t happen during Annihilydra‘s turn, he can then use his other actions as normal. Everyone else’s HP and Spell Slots remain as they were before the rewind. 

No Mythic Actions.