Not to be confused with the Suspicious Looking Skull, a vanilla old console exclusive boss summoning item that summons Ocram
The Suspicious Skull is a boss summoning item which summons Skeletron without killing the Clothier. It is sold by the Mutant.
Using it during the day will instead spawn the Dungeon Guardian. If it is used before defeating Skeletron, it will set the time to the start of the night to give you more time.