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Huge monstrosity, neutral evil
Multiattack. The behir makes two attacks: one with its bite and one to constrict.
Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 22 (3d10 + 6) piercing damage.
Constrict. 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 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 20 feet long and 5 feet wide. Each creature in that line must make a DC 16 Dexterity saving throw, taking 66 (12d10) lightning damage on a failed save, or half as much damage on a successful one.
Swallow. The behir makes one bite attack against a Medium 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 21 (6d6) acid damage at the start of each of the behir's turns. A behir can have only one creature swallowed at a time.
The serpentine behir crawls along floors and clambers up walls to reach its prey. Its lightning breath can incinerate most creatures, even as more powerful foes are constricted in its coils and eaten alive.
Cavern Predators. Behirs lair in places inaccessible to other creatures, favoring locations where would-be intruders must make a harrowing climb to reach them. Deep pits, high caves in cliff walls, and caverns reached only by narrow, twisting tunnels are prime sites for a behir ambush. A behir's dozen legs allow it to scramble through its lair site with ease. When not climbing, it moves even faster by folding its legs beside its body and slithering like a snake.
Foes of the Dragons. In times long forgotten, giants and dragons engaged in seemingly endless war. Storm giants created the first behirs as weapons against the dragons, and behirs retain a natural hatred for dragonkind.