Monsters in the Afterlife may drop coffins, or these may be found on the ground. A coffin is like a battered chest and can be unlocked to gain access to items found inside.
Coffins will predominantly provide resources for the Hag/new class to use in the Charms and Hexes skill although they may provide additional items for all classes to use or the coffins themselves may be used in some form of trophy quest.
Also Sarcophagi may drop from monsters based on the Lifestream, the harder the difficulty of the monsters based on the kills in the Lifestream the higher the chance of getting a sarcophagus.
The difference between Sarcophagi and Coffins is that Sarcophagi have a higher chance of dropping the Charm and Hexes ingredients.