A Casket (hard) is received after from completing the final clue scroll of a hard clue scroll trail, and contains the reward for the trail. Multiple rewards can be found in the chest. It counts for having a clue scroll in the player's bank and thus players can not have more than one casket at a time. The rewards from the casket can come up to six items at a time.
Treasure Trail NPCs, such as Uri, may give you one. You can also receive a casket by digging at a map spot or after completing a Coordinate clue.