15th Anniversary party hat was removed from RuneScape after an update, but still exists in-game for those who had obtained it.
The 15th Anniversary party hat is a party hat which could be created by players who collected 100 party hat fragments during the 15 Year Anniversary Celebrations. The fragments could be received by popping the balloons dropped at all lodestones (except for Prifddinas and the 2015 Christmas event lodestones). The fragments were received 2 at a time.