The peryton is a monstrosity with the body of an enormous bird of prey and the head of a stag, albeit a carnivorous stag with nasty incisors. Perytons roost in mountains and rocky hills near settled areas where they can find prey. According to the Monster Manual, they favor humanoids in general and humans and elves in particular, and they’ll often try to rip out their prey’s heart and carry it off. “When attacking a humanoid,” the MM says, “a peryton is single-minded and relentless, fighting until it or its prey dies.” This is the behavioral oddity that distinguishes it, as a monstrosity, from an ordinary evolved creature.
All of a peryton’s physical abilities are above average, but its Strength is especially high, so it’s going to go for big-damage attacks, either by dive-bombing its prey or in a close-quarters scrap. Its features include Dive Attack and Flyby: the former is an aerial charge that deals additional damage, while the latter exempts it from opportunity attacks when it flies out of an enemy’s reach. The peryton has a flying speed of 60 feet, and its Dive Attack requires it to fly 30 feet to gain the extra damage.
This combination makes the peryton’s preferred attack tactic obvious: Its first attack will always be a Dive Attack, from a distance of exactly 30 feet if possible. It uses 30 feet of its move to conduct this attack; Multiattacks (action) with Talons and Gore, gaining Dive Attack damage on both of these; then, if its target is still alive and kicking, uses the other 30 feet of its move to fly away again. As long as its prey lives, it will repeat this half-move/Dive Attack/half-move combination.
What will make a peryton deviate from this pattern? Well, if it succeeds in rendering its target unconscious, and it’s not taking damage from a magical weapon (more on this in a moment), it may keep attacking its target until he or she is not merely unconscious but dead. At that point, it will have torn out its prey’s heart, and it will proceed to fly off with it. Alternatively, if the target is of Small size, it may simply pick the unconscious target up in its talons and carry him or her away to its nest.
Perytons have resistance to physical damage from nommagical weapons, so when they’re struck by magical weapons, they take notice. If a peryton has already rendered a target unconscious, but it’s taking damage from a magical weapon, it won’t remain on the ground with its prey. Instead, it will continue its pattern of flying up out of reach in between attacks, and it will refocus its attacks on whoever’s wielding that weapon. Perytons aren’t super-bright, but if someone’s shooting one with a ranged weapon, it can figure out where that attack is coming from, and if it happens to be a magical ranged weapon, that just might be enough to drive the peryton off … if it’s alone. However, perytons often hunt in mated pairs, and not only will a pair of perytons not be scared off by Owena’s magic bow, they’ll ferociously focus their attacks on her, from two opposite directions (see the optional Flanking rule on page 251 of the Dungeon Master’s Guide, which I recommend always using).
When they’re not focusing their attacks on a single antagonist in this way, perytons don’t fight cooperatively. Instead, each peryton in a group (I’d have them only appear alone, in pairs or—on rare occasions, when necessary for encounter balance—in a group of four consisting of two mated pairs) will pick out a separate target to focus on, because each is after its own meal.
When it’s got its sights on a humanoid victim, the only thing that will drive a peryton away is being seriously injured (reduced to 13 hp or fewer) with magical weapons contributing at least 10 hp of the damage it takes. Even then, it will continue to reconnoiter its victim (and the possessor of the magical weapon), and it will return and attack again when it’s feeling better. However, a peryton that gets what it came for—either a humanoid heart or an entire Small humanoid that it can fly away with—will take its prize and leave, even if it’s not wounded at all. If one of a mated pair succeeds in tearing out its victim’s heart, its mate will depart along with it when it leaves.
Finally, note that perytons have keen senses and a high Perception skill—they’re hard to hide from—but they don’t have darkvision. Perytons are daytime hunters, making them good candidates for a random encounter on the road.