When you kill a monster, you get one level per monster, unless the monster card says something else, and you get all its Treasure!
Each monster has a Treasure number on the bottom of its card. Draw that many Treasures, modified by any monster enhancers played on it.
Draw face-down if you killed the monster alone. Draw face-up, so the whole party can see what you got, if someone helped you, even if they aren’t taking any Treasures.
If you defeat a monster by nonlethal means, you do not get a level and you may or may not get the Treasure, depending on the method.
Treasure cards can be played as soon as you get them, even if you are the helper.