Casket (easy) 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 easy reward caskets. For the current iteration that exists in game now, see Reward casket (easy).
A Casket (easy) is received after from completing the final clue scroll of an easy clue scroll trail, and contains the reward for the trail, unless you're doing 2 clues at once by droptricking, it will then possibly contain another clue.
The casket may appear depending on specific circumstances. If the last clue tells you to dig or do an emote, you will be given a casket upon completion. If it tells you to search in a crate, barrel, etc., you will receive the reward instead.