The rusty casket is a quest item found by the player during the Horror from the Deep quest after killing the Dagannoth Mother. After completing the quest, players can take this to Jossik by going up the stairs of the lighthouse and he will tell them something is written on it. Depending on which answer they give, he will give them a damaged prayer book in one of six colours, which correspond to the six major gods of Gielinor. The pages for these books can be received from all four levels of Treasure Trails. Note that if you dropped it and wished to pick it back up, it'll say "It is too old and fragile to pick up."
Should you lose this book Jossik will give you the old book back. Please note: All pages previously put in these books will still be there.