Crossbreed Hydra of the Ruined Sanctum

Hold Breath. The hydra can hold its breath for 1 hour.

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

Whenever the hydra takes 25 or more damage in a single turn, one of its heads dies. If all its heads die, the hydra 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 damage from a corresponding type since its last turn (red is weak to cold, yellow to radiant, blue to fire). The hydra regains 10 hit points for each head regrown in this way.

If the hydra has multiple heads of the same color, one of those heads may make a breath attack with the other(s) partake in the hydra's multiattack.

Elemental Affinity.  The hydra is resistant to fire damage while it retains a red head, cold while retaining a blue head, and necrotic while retaining a yellow head. Resistance is ignored in the case of cold damage specifically targeting a red head, or fire damage targeting a blue head.

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

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

Multiattack. The hydra makes as many bite attacks as it has heads.

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

Fire Breath (Recharge 5-6). The hydra exhales fire in an 30-foot line that is 5 feet wide. Each creature in that line must make a DC 15 Dexterity saving throw, taking 36 (8d8) fire damage on a failed save, or half as much damage on a successful one.

Cold Breath (Recharge 5-6). The hydra exhales an icy blast in a 30-foot line that is 5 feet wide. Each creature in that line must make a DC 15 Dexterity saving throw, taking 18 (4d8) fire damage on a failed save, or half as much damage on a successful one. On a failed save, the target is also frozen in a solid block of ice. They may make a DC 15 Strength saving throw on the beginning of their turn, freeing themselves on a success. The ice block has 10 AC, 15 HP, and is instantly destroyed by fire damage.

Radiant Breath (Recharge 5-6). The hydra exhales radiant light in an 30-foot line that is 5 feet wide. Each creature in that line must make a DC 15 Dexterity saving throw, taking 27 (6d8) radiant damage and being blinded for one turn on a failed save, or half as much damage on a successful one.