Ancient Behir

Legendary Resistance (3/Day). If the behir fails a saving throw, it can choose to succeed instead.

Multiattack. The behir makes three attacks: one with its bite, one to constrict, and one with its tail.

Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 30 (4d10 + 8) piercing damage plus 7 (2d6)

Constrict. Melee Weapon Attack: +14 to hit, reach 5 ft., one Huge or smaller creature. Hit: 24 (3d10 + 8) bludgeoning damage plus 17 (3d10 + 8) slashing damage. The target is grappled (escape DC 18) if the behir isn't already constricting a creature, and the target is restrained until this grapple ends.

Tail. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 17 (2d8 + 8) bludgeoning damage.

Lightning Breath (Recharge 5–6). The behir exhales a line of lightning that is 40 feet long and 10 feet wide. Each creature in that line must make a DC 20 Dexterity saving throw, taking 82 (15d10) lightning damage on a failed save, or half as much damage on a successful one.

Swallow. The behir makes one bite attack against a Large or smaller target it is grappling. If the attack hits, the target is also swallowed, and the grapple ends. While swallowed, the target is blinded and restrained, it has total cover against attacks and other effects outside the behir, and it takes 28 (8d6) acid damage at the start of each of the behir's turns. A behir can have only one creature swallowed at a time.

If the behir takes 40 damage or more on a single turn from the swallowed creature, the behir must succeed on a DC 22 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.

The behir can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The behir regains spent legendary actions at the start of its turn.
Detect. The behir makes a Wisdom (Perception) check.
Move. The behir moves up to half its speed.
Tail. The behir makes a tail attack.