Fire Splash. When the Abyssal Behir takes damage of any type but psychic, each creature within 5 feet of the Abyssal Behir takes 9 (2d8) fire damage.
Ambusher. The Abyssal Behir has advantage on attack rolls against any creature it has surprised.
Hellfire Maw. This targets any creature currently swallowed and that has taken any damage inside of the Abyssal Behir. A creature imprisoned in this manner has disadvantage on death saving throws. If it dies while imprisoned, the Abyssal Behir regains 30 hit points and gains an additional action on its next turn. Additionally, at the start of its next turn, the Abyssal Behir can regurgitates the slain creature as a bonus action, and the creature becomes an undead.
If the victim had 2 or fewer Hit Dice, it becomes an Ash Zombie. If it had 3 to 5 Hit Dice, it becomes a Husk Zombie. Otherwise, it becomes a Ghoul. When the creature dies in this manor the soul is washes up next to the River Syx on a top layer of the Abyss or Avernus.
Multiattack. The Abyssal 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 and 12 (2d8 + 3) fire 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 Abyssal Behir isn't already constricting a creature, and the target is restrained until this grapple ends. Any restrained creature at the start of its next turn takes 9 (2d8) fire damage.
Swallow. The Abyssal 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 Abyssal Behir, and it takes 18 (4d8) acid damage plus 9 (2d8) fire damage at the start of each of the Abyssal Behir's turns. An Abyssal Behir can have only one creature swallowed at a time.
If the Abyssal Behir takes 30 damage or more on a single turn from the swallowed creature, the Abyssal 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 Abyssal Behir. If the Abyssal 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.
Inferno Lighting Breath (Recharge 5–6). The Abyssal Behir exhales a line of lightning and flames 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 plus 22 (3d10 + 6) fire damage on a failed save, or half as much damage on a successful one.
Aggressive. As a reaction, the Abyssal Behir can move up to half of its walking speed toward a hostile creature that it can see.
Description
The serpentine Abyssal Behir crawls along floors and clambers up walls to reach its prey. Its lightning/fire breath can incinerate most creatures, even as more powerful foes are constricted in its coils and eaten alive. These Abyssal Behir have fed on abyssal touched creatures and/or live next to an open portal to the Nine Hells and/or The Abyss where the continual energy that corrupts the nearby land has also infect these creatures. Either way their normal appearance has been corrupted and defiled.
An Abyssal Behir's monstrous form resembles a combination of centipede and crocodile. Abyssal Behir's jagged scaled hide color ranges from deep blue to charcoal black or deep red browns, fading to pale yellow on its underside
Cavern Predators. Abyssal Behir 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 Abyssal Behir ambush. An Abyssal 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.
Abyssal Behir swallow their prey whole and instead of enter a period of dormancy while they digest like normal behirs, the internal hellflames begin to corrupt and consume their prey. The Abyssal Behir will continue to seek at more prey to consume as the hellflames have created the unsatiated hunger for this corrupted creature. Abyssal Behir attempts to keep its food inside of its body as long as it can to attempt to feel less hungry, but it always fails. After a certain time or if under attack, the Abyssal Behir will regurgitate the corrupted body out side of its den where it will become an undead creature most of the time. Once the prey dies inside of the Abyssal Behir, the soul is transported to be washed up next to the River Syx on a top layer of the Abyss or Avernus.
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.
A behir never makes its lair in an area it knows to be inhabited by a dragon. If a dragon attempts to establish a lair within a few dozen miles of a behir’s lair, the behir is compelled to kill the dragon or drive it off. Only if the dragon proves too powerful to fight does a behir back down, seeking out a new lair site a great distance away.