The casket (also known as One-Eyed Hector's chest) is a reward from the Pirate's Treasure quest. It is found by digging with a spade next to the statue of Saradomin in Falador Park. A gardener will spawn and attempt to attack the player who digs there, and he must be killed. Once the casket is received, the quest ends. It can be opened to receive 450 coins, an emerald, and a gold ring.
It was buried by Hector, and the pirate message leads the player to the location of the treasure.