The top of sceptre is a piece of the skull sceptre that is dropped by Catablepon on the third level of the Stronghold of Security. It is combined with the bottom of sceptre to get the runed sceptre. After combining a message in chatbox will state: "The two halves of the Sceptre fit perfectly. The Sceptre appears to be designed to have something on top." It is usually considered the most difficult part of the skull sceptre to obtain. The Stronghold of Security is the dungeon beneath Barbarian Village, to the west of Varrock and east of the Dwarven Mine.
For an exhaustive list of all known sources for this item, see here