Multiattack. The behir makes two attacks: one with its bite and one to constrict.

Bite. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 15 (2d10 + 4) percing damage.

Constrict. Melee Weapon Attack: +7 to hit, reach 5 ft., one Medium or smaller creature. Hit: 9 (1d10 + 4) bludgeoning damage plus 9 (1d10 + 4) slashing damage. The target is grappled (escape DC 14) if the behir isn't already constricting a creature, and the target is restrained until this grapple ends.

Lightning Breath (Recharge 5-6). The behir exhales a line of lightning that is 15 feet long and 5 feet wide. Each creature in that line must succeed on a DC 14 Dexterity saving throw, taking 49 (9d10) lightning damage on a failed save, or half as much damage on a successful one.