Urium remains are an item found and used in the Shades of Mort'ton minigame. To obtain these remains, the player must kill a Urium Shade, located behind the gold door in the Shade Catacombs.
To burn the Urium remains, first place redwood pyre logs on a funeral pyre, followed by the Urium remains, then ignite the pyre with a tinderbox. This will grant the player 500 Firemaking experience, and 120 Prayer (180 with Morytania Diary 3 complete) experience.
Urium remains will always reward the player with a gold shade key, or some coins. Gold keys give unrestricted access to all areas of the shade tombs and opens the gold chests in the Urium area of the tombs, which give the best rewards out of all the chests.
Completing the Morytania Diary 3 offers 50% more Prayer experience, and completing the Morytania Diary 4 offers 50% more Firemaking experience.
Drop rates estimated based on 6,000 cremations from the Drop Rate Project, unless otherwise cited.
For an exhaustive list of all known sources for this item, see here