Sliskelion XP lamp was removed from RuneScape after an update, but still exists in-game for those who had obtained it.
Sliskelion XP lamp is a lamp reward for handing in 50 Sliskelion pieces at the God Scoreboard. This lamp can only be obtained once a week. This lamp can be used on any skill and gives experience equivalent to that of a Large XP lamp. It is possible to have more then 1 Sliskelion XP lamp at a time.