Riyl remains are an item found and used in the Shades of Mort'ton minigame. To obtain these remains, the player must kill a Riyl shade, located in the underground tomb. (Note:To enter the tomb, the player needs at least a bronze shade key, obtainable by killing and cremating a Loar shade.) To burn the Riyl remains, the player needs some pyre logs (see table for more info) and a tinderbox. First place the pyre logs on a funeral pyre, followed by the Riyl remains, then ignite the pyre. This will grant the player Firemaking experience (see table), and 59.5 Prayer experience.
Once burned, a steel or black shade key (or some coins) will appear next to the pyre. The keys can be used to either enter the tomb, or open the corresponding coloured doors to chests containing valuable items.
Completing the Morytania Diary 3 offers 50% more prayer xp. Completing the Morytania Diary 4 offers 50% more firemaking xp.
Players may only use Willow pyre logs or better to burn Riyl remains.
For an exhaustive list of all known sources for this item, see here