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 three colours, which correspond to the three major gods of Gielinor: Guthix, Saradomin, and Zamorak. Note that players cannot choose the books for Armadyl, Bandos, and Zaros until they complete one god book post-quest.
The pages for these books can be received from all five 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., and Telekinetic Grab cannot be cast on it.
Should you lose this book Jossik will give you the old book back. All pages previously put in these books will still be there.
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