Fenrir close to direwolves cause the direwolves' eyes to glow, and gives them a buff which reduces incoming damage by ~12.5% and increases damage output by ~25%. This buff can stack with mate boost, pack bonus, howl, and yuti roar.
Very useful for raising wyverns and drakes because fenrir's inventory acts like a preserving bin/fridge making the milk or venom last 2 days in its inventory
Only one person in the fight will get a cryo pod with a Fenrir in it once the boss dies, and the level depends on which difficulty fight you pick. (On official)