Thessalhydra

Huge Monstrosity, Unaligned

AC: 14

Initiative: +1 (11)

HP: 69 (6d12 + 30)

Speed: 30 ft., swim 30 ft.

STR: 19 (+4)
DEX: 12 (+1)
CON: 20 (+5)
INT: 5 (-3)
WIS: 10 (+0)
CHA: 7 (-2)

Skills: Perception +4

Immunities: acid

Condition Immunities: blinded; charmed; deafened; frightened; stunned

Senses: darkvision 60 ft., Passive Perception 14

CR: 4 (XP 1,100; PB +2)

Actions

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.

Legendary Actions

Legendary Action Uses: 2. Immediately after another creature’s turn, this creature can expend a use to take one of the following actions; it regains all expended uses at the start of each of its turns.

Detect.

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

Tail Swipe.

The thessalhydra makes a tail pincer attack.

Find this creature in the encounter builder

Source: Hunt for the Thessalhydra, p. 41 - Licenses and attribution