Behir

Spider Climb. The behir can use its climb speed even on difficult surfaces and upside down on ceilings.

Serpentine. The behir can move through a space as narrow as 5 feet wide, vertical or horizontal, at full speed, without squeezing.

Multiattack. The behir makes a bite attack and then a constrict attack.

Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 24 (3d12 + 5) piercing damage. If the target is a Medium or smaller creature grappled by the behir and the behir has not swallowed anyone else the target is swallowed. A swallowed creature is blinded and restrained it has total cover from attacks from outside the behir and it takes 21 (6d6) acid damage at the start of each of the behirs turns.

If a swallowed creature deals 30 or more damage to the behir in a single turn, or if the behir dies, the behir vomits up the creature.

Constrict. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 21 (3d10 + 5) bludgeoning damage. The target is grappled (escape DC 17) and restrained while grappled. The behir can grapple two creatures at once.

Lightning Breath (Recharge 5-6). The behir breathes a line of lightning 5 feet wide and 20 feet long. Creatures in the area make a DC 16 Dexterity saving throw taking 56 (16d6) lightning damage on a failed save or half damage on a success.

Vengeful Breath (1/Day. When struck by a melee attack, the behir immediately recharges and uses Lightning Breath, including the attacker in the area of effect.