Acidic Blood. When the Drakkenhydra takes piercing or slashing damage, each creature within 5 feet of it takes 11 (2d10) acid damage.

Fire Resistant. Years of living in water contaminated by delerium sludge have mutated the Drakkenhydra, giving it resistance to fire.

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

Legendary Resistance (3/Day). If the Drakkenhydra fails a saving throw, it can choose to succeed instead.

Multiple Heads. The Drakkenhydra has multiple heads. While it has more than one head, it has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, or knocked unconscious.

The Duchess has had her fishfolk minions cause the Drakkenhydra to grow additional heads. When the party encounters it, the Drakkenhydra will have heads equal to 1 plus two times the highest proficiency bonus found among the party members (5, 7, 9, 11, 13).

Whenever the Drakkenhydra takes 40 or more damage in a single turn, one of its heads is severed. If all of its heads die, the Drakenhydra dies. When a head is severed, it will thrash about for one round as it regrows a short tail and begins crawling toward the nearest source of contaminated water at a speed of one foot per round. A severed head has 1 hit point, and if a head escapes, it can regrow into a five-headed Drakkenhydra in 7 (2d6) months.

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

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

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

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

Bite. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 17 (2d10 + 6) piercing damage.

Spit Acid (Recharge 5–6). Each of the Drakkenhydra's heads can spit acid at a creature it can see within 45 feet. The target must make a DC 15 Dexterity saving throw, taking 17 (2d10 + 6) acid damage on a failed save, or half as much damage on a successful one.