Multiattack. The thessalhydra makes one maw attack and one Flurry of Bites.

Flurry of Bites. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit:7 (1d6 + 4) piercing damage plus 10 (4d4) poison damage.

Maw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit:9 (1d10 + 4) piercing damage plus 5 (1d10) acid damage.

Tail Pincer. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit:10 (1d12 + 4) slashing damage, and the target is grappled. As an action, the target can escape the grapple by succeeding on a DC 14 Strength (Athletics) or Dexterity (Acrobatics) check (its choice). Until this grapple ends, the thessalhydra can't use its tail pincer.

Acid Saliva (Recharge 5–6). The thessalhydra spits a glob of acid at a point it can see within 30 feet of it. Each creature within 10 feet of that point must make a DC 15 Dexterity saving throw, taking 18 (4d8) acid damage on a failed save, or half as much damage on a successful one.

Detect. The thessalhydra makes a Wisdom (Perception) check with advantage.

Tail Swipe. The thessalhydra makes a tail pincer attack.