Inside the corpse of some specific creatures and in the Reward Chest.
Holds the loot of a specific creature you helped defeat. Contents will depend on your participation while fighting it. A container will also appear inside the Reward Chest if not looted before.
A list of the creatures using this system can be found on the Reward Chest page.
Important: a Reward Container will be deleted 7 days after its creation, so make sure to loot it before that happens.