Clockwork Horrydra

Multiple Heads. The hydra has ten heads. While it has more than one head, the hydra has one extra layer of advantage on saving throws against being blinded, deafened, stunned, and knocked unconscious.

Whenever the hydra takes 50 or more damage in a single turn, it must make a DC 15+(damage dealt-50) Constitution saving throw, or one of it's head shuts down. If all its heads die, the hydra explodes, forcing all creatures within 120 feet to make a DC 25 Dexterity saving throw or take 75 (25d6) fire damage, or half on a success. When the Horrydra takes a long rest, the heads will come back online, at full health.

Reactive Heads. For each head the hydra has beyond one, it gets an extra reaction that can be used only for opportunity attacks or legendary actions.

Lightning Absorption. If the Horrydra is subject to lightning damage, the damage is negated, and the Horrydra is instead healed for the damage dealt.

System Shutdown. If the Horrydra is subject to more than 250 damage in the span of an hour, it must succeed on a DC 28 Constitution saving throw or fall unconscious for one round. If this happens twice, the DC is reduced to 18. It if rolls a critical either time, it is not knocked unconscious, and regains 5d20+50 HP.

Multiattack. The hydra makes as many bite attacks as it has heads, and can use one or both of it's tails.

Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 25 (3d10 + 10) piercing damage.

Stomp. The Hydra can move up to it's walking movement in any direction, and any creatures of large or smaller size must make a DC 25 Dexterity saving throw or take 19 (3d6+10) bludgeoning damage if it stopped on the target, or 38 (6d6+20) bludgeoning damage if it moved through the target.

Tail. Melee Weapon Attack: +10 to hit, reach 15 ft., 10 ft. x 10 ft. square centered on 1 target. Hit: 19 (3d6+10) bludgeoning damage, and target(s) must make a DC 25 Strength saving throw or be knocked away up to 10 feet and prone.

The Horrydra is a legendary creature that can has 10 legendary actions, which it can use at the end of another creature's turn. It regains spent legendary actions at the start of it's next turn.

Bite (Costs At Least 1 Action). The Hydra makes any amount of bite attacks, as long as it has enough actions remaining to use however many it pleases.

Sweep (Costs 2 Actions). The Hydra makes two tail attacks against two separate targets.

Furious Multiattack (Costs 10 Actions). The Horrydra can use it's stomp attack, all ten of it's head's bite attacks, and both of it's tail attacks. The Horrydra cannot use this action again until it finishes a long rest.