Casket (hard) was removed from RuneScape after an update, but still exists in-game for those who had obtained it.
This article is about the old version of the hard reward caskets. For the current iteration that exists in game now, see Reward casket (hard).
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. 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. There can be up to six rewards at a time in the casket.
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.