Arca-Hydra

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

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

Whenever the Arca-hydra takes 25 or more damage in a single turn, one of its heads dies. If all its heads die, the Arca-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 fire damage since its last turn. The Arca-hydra regains 10 hit points for each head regrown in this way.

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.

Acidic Blood. When the Arca-hydra takes piercing or slashing damage, each creature within 5 feet of the Arca-hydra takes 4 (1d8) acid damage.

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.

Arcane Breath (Recharges 5-6). The Arca-hydra expels one of the following effects from one of its heads.

Oily Discharge (1 or 2 on d6) - The Arca-hydra spits a glob covers the ground in a 10-foot square centered on a point within 30 feet and turns it into difficult terrain for the duration.

When the grease appears, each creature standing in its area must succeed on a DC 15 Dexterity saving throw or fall prone. A creature that enters the area or ends its turn there must also succeed on a Dexterity saving throw or fall prone.

Colorful Breath (3 or 4 on d6) - A dazzling array of flashing, colored light sprays from the Arca-hydra's mouth. Roll 12d10; the total is how many hit points of creatures this spell can affect. Creatures in a 30-foot cone originating from one of the Arca-hydra's heads are affected in ascending order of their current hit points (ignoring unconscious creatures and creatures that can't see).

Starting with the creature that has the lowest current hit points, each creature affected by this spell is blinded until the end of your next turn. Subtract each creature's hit points from the total before moving on to the creature with the next lowest hit points. A creature's hit points must be equal to or less than the remaining total for that creature to be affected.

Fiery Breath (5 or 6 on d6) - The Arca-hydra exhales fire in a 30-foot cone. Each creature in that area 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.

The fire ignites any flammable objects in the area that aren't being worn or carried