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

Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 22 (3d10 + 6) piercing damage.

Grab. Melee Weapon Attack: +10 to hit, reach 5 ft., one Large or smaller creature. Hit: 17 (2d10 + 6) bludgeoning damage plus 17 (2d10 + 6) slashing damage. The target is grappled (escape DC 16) if the dragon isn't already grabbing a creature, and the target is restrained until this grapple ends.

Shadow Breath (Recharge 5–6). The dragon exhales a line of shadow that is 20 feet long and 5 feet wide. Each creature in that line must make a DC 16 Dexterity saving throw, taking 66 (12d10) necrotic damage on a failed save, or half as much damage on a successful one.