Behir

A behir ally is unusual but not impossible. Behirs hate dragons, and a dragon-hunting party that saves a behir's brood from a young blue dragon's incursion can earn one favor, paid in full, never repeated. The behir does not befriend; it acknowledges a debt and discharges it. Make the deal in Draconic, the only language it speaks, and have the party's Draconic-speaker translate every line.

In combat, the behir is a delivery system for one Lightning Breath. Position the party out of the 90-foot line, point at the cluster of enemies, and let it fire. 12d10 lightning at DC 16 Dex clears a battlefield of mooks before the rogue has acted. After that, it Multiattacks the biggest target: Bite at +10 for 30 damage, Constrict at DC 18 Str to grapple and restrain, then Swallow as a bonus action on the next turn. A swallowed BBEG is essentially out of the fight, taking 21 acid a turn with no line of sight to their own party.

The behir will not protect the PCs. It will not coordinate. If the fight goes badly for the party but well for the behir, the behir finishes its kill and leaves. Once the agreed enemy is dead, the debt is paid, and the behir climbs out the way it came. Anyone who tries to keep it around or push for a second favor learns why behirs eat humanoids.

Have the behir name a single condition for the favor: do not bring dragons into its tunnels again. The party either keeps that promise or the next behir scene is the adversary tab.

A behir is an ambush predator built like a centipede with a dragon's head, and it fights from above. Climb 50 feet means it should be on the cavern ceiling or coiled around a stalactite when the party arrives, not standing in the middle of the room. Stealth +7 against a Passive Perception that ignores it (Darkvision 90, Perception +6) means the party walks in, sees nothing, and rolls initiative when the lightning hits.

Open with Lightning Breath. 12d10 in a 90-foot line by 5 feet wide, DC 16 Dex, averages 66 damage. Position the line down a corridor or marching column so it catches three or four PCs at once. The DC is the soft spot of this stat block (the casters will probably save), but a paladin in plate at +1 Dex eats the full hit. After the breath, drop from the ceiling and use Multiattack: one Bite at +10 for 30 damage (piercing plus lightning), and Constrict at DC 18 Str for 28 bludgeoning, Grappled at escape DC 16, Restrained until the grapple ends. On the next turn, Swallow as a bonus action against the grappled target. DC 18 Dex on the failure puts a PC inside the behir taking 21 acid a turn with Total Cover from outside help.

Manage the swallow risk before you use it. The behir can hold one creature at a time, and 30 damage in a turn from inside forces a DC 14 Con regurgitate. A barbarian raging with a greataxe will pop back out, prone, in one turn. A wizard will not. Pick your meal accordingly. If Lightning Breath recharges and the rest of the party has spread out, drop the swallowed PC, breathe again, and start over.

Behirs are cowards when outmatched. Once Bloodied with no breath weapon online, climb back up to the ceiling and slither into a crack the party cannot follow. Leave the swallowed PC behind on the floor, half-digested. The party gets a rescue scene, not a corpse.