Before an update in early 2009, the black scimitar was behind a locked door on the 1st2nd floor[UK] of Ardougne Castle which needed to be picked, requiring 61 Thieving. The door was moved further east to reach a Thieving Paladin chest. Furthermore, the black scimitar used to be located on a bed until it was removed on the 14 August2012.
How to steal the black scimitar:
Search the doors on the floor King Lathas is on. It may take several tries to open the door.
Search the chest for traps. If it says "The chest has already been looted", it means someone else has already collected it or is empty.
Search the other room (if you got the wrong chest). Search it carefully for traps, as it can hit for 100-300 damage if not searched. This requires 72 Thieving.