The behir is, simply, a predator. A huge monstrosity, in the neighborhood of 15 feet long, it’s fast, tough and strong, able to run as fast as a lion and to climb faster than most humanoids can run. Its Intelligence is at the upper end of animal range, and it’s a good judge of prey. It’s also surprisingly stealthy, able to strike from ambush and do spectacular damage to its victims.
But all this power comes at a cost: Behirs get sleepy as they digest their prey. Because of this, once a behir strikes and swallows its prey, it immediately breaks off and retreats to its lair or some other hiding place so that it can absorb its meal in peace.
Normally, a creature with a recharge ability prefers it over any other method of attack, but in the case of behirs, Lightning Breath is an oddly limited-use feature. Its range is only 20 feet, meaning that in most cases, it’s only likely to strike a single target (see “Targets in Area of Effect,” Dungeon Master’s Guide, page 249), and while the damage it does is substantial—about 50 points of expected damage—it doesn’t help the behir eat anything.
In contrast, the behir’s Multiattack comprises a Constrict action, which does an average of 34 points of damage and restrains its target on a hit, and a Bite action, which does an average of another 22 points of damage, with advantage on the attack if the target is restrained. That’s 56 points of damage right there, and the behir’s chance to hit is extremely high, compared with its chance of doing full damage with its Lightning Breath. Finally, on its next turn, it can Swallow the creature it’s restraining, which is what it came for.
Because of this, I see the behir’s Lightning Breath as more of a self-defense feature than an attack option, which probably won’t come into play unless and until it’s moderately wounded (reduced to 117 hp or fewer). If a behir uses this action, it will certainly reposition itself first in order to strike two targets (or more, if possible) in its area of effect. This requires the targets to be within 20 feet of each other and the farthest target to be no more than 20 feet from the behir.
As phrased in the Monster Manual, the behir’s Multiattack is defined as “two attacks: one with its bite and one to constrict,” but I’m assuming this is just alphabetical order, because constricting first, then biting makes much more sense. A successful Constrict action gives the behir advantage on its follow-up Bite. A successful Bite, on the other hand, does nothing to improve its chance of succeeding at a subsequent Constrict.
This entry is one of the few in the MM in which the flavor text genuinely enhances the reader’s understanding of the stat block. I’m referring specifically to the paragraphs headed “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. . . . Behirs swallow their prey whole, after which they enter a period of dormancy while they digest.
OK, so we can imagine a behir lying in wait, concealed in the darkness of a subterranean cavern or among rocks on a cliffside (twilight is ideal, because of its darkvision), for potential prey to pass by. When it does, the behir pounces, lunging forth from its hiding place (i.e., advantage from unseen attack), using its Constrict ability to grapple and restrain the target creature, and following up with a Bite (with advantage, if the Constrict attack succeeded).
On its following turn, if the behir’s target hasn’t escaped the grapple, it uses the Swallow action (which it has advantage on, because the target is still restrained) to try to gulp down its target. If it succeeds, it immediately uses its full movement to scramble away, preferably in a direction where its victim’s allies can’t follow.
Note that a behir can Constrict a Large or smaller creature, but it can only Swallow a Medium or smaller creature. A quick-thinking spellcaster can preserve an ally (or him- or herself) from being Swallowed by snapping off an enlarge spell. Funny thing about being restrained: It doesn’t interfere with the gestures and movements necessary to cast a spell with a somatic requirement!
A behir without a grappled target simply tries its Constrict/Bite Multiattack again—it’s not smart enough to change it up. Only if it’s taken enough damage to realize it needs to defend itself (see above) will it introduce Lightning Breath into the mix, using this action whenever it’s not on cooldown and Multiattacking whenever it is.
Once a behir is seriously injured (reduced to 67 hp or fewer), it says, “Fine, forget this,” and retreats. A behir’s armor class is high enough, and its hit points numerous enough, that it never concerns itself with opportunity attacks from a single attacker, simply Dashing away. But if it’s within reach of more than one, it will Dodge until it’s out of range, then Dash. This applies whether it’s running away because of its injuries or because it’s already had its dinner.