Bite. Melee Weapon Attack. Reach 10 ft; 1d20+5 to hit, does 1d8+3 Piercing damage plus 1d4 Lightning damage to one target.

Constrict. Melee Weapon Attack. Reach 5 ft; can only target one Large or smaller creature if a bite attack succeeds. 1d20+1 to hit, does 1d8+3 Bludgeoning damage plus 1d4 Lightning damage; target is grappled by it (escape DC 13; disadvantage) if the behir isn't already constricting a creature.

Swallow. Melee Weapon Attack. can only target one Large or smaller creature if the target is already Constricted. 1d20+1 to hit. On success, the target takes the bite damage and is swallowed. The grapple ends and the target is now blinded and restrained, taking 2d6 Acid damage at the start of each of the behir's turns.

If the behir takes 30 damage or more on a single turn from the swallowed creature, the behir must succeed on a DC 14 Constitution saving throw at the end of that turn or regurgitate the creature- which falls prone in a space within 10 feet of the behir. If the behir dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.

Lightning Breath. The behir exhales a line of lightning (10 ft X 5 ft). Each creature in line must make a DC 13 Dexterity saving throw, taking 2d8+1 lightning damage on a failed save- or half as much damage on a successful one; Recharge. 1d6 (Recharge 5-6)