King Barrel is a character in the Amnesia Custom Story: PewDiePie's Revenge.
As its name implies it is the leader of the barrels. It seems to be three times the size of an average barrel.
PewDie found him in the Throne Room, obviously on the throne.
While a normal dagger could kill a regular barrel, it couldn't kill the king. It could only be killed by a so-called "Master Dagger."