Archydra

Hold Breath. The archydra can hold its breath for 4 hours.

Multiple Heads. The archydra has five heads. While it has more than one head, the archydra has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.

Whenever the archydra takes 25 or more damage in a single turn, one of its heads dies. If all its heads die, the archydra dies.

At the end of its turn, it grows two heads for each of its heads that died since its last turn, unless it has taken fire damage since its last turn. The archydra regains 10 hit points for each head regrown in this way.

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

Wakeful. While the archydra sleeps, at least one of its heads is awake.

Multiattack. The archydra makes as many bite or arc lightning attacks as it has heads.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 10 (1d10 + 5) piercing damage.

Arc Lightning. Ranged Spell Attack: +8 to hit, ranged 120 ft., one target. Hit: 10 (1d10+5) lightning damage.

Lightning Storm (Recharge 6). The archydra exhales blasts of lightning in a 60-foot cone. Each creature in that cone must make a DC 14 Dexterity saving throw, taking 5 (1d10) lightning damage per head on a failed save, or half as much damage on a successful one.