You awake in a dark castle, hearing eldritch howling. Investigating, you follow the noises to a massive door in the lowest dungeon. The door is covered with magical signs. There’s blood everywhere, from four knights and a wizard lying dead on the floor. Opened padlocks hang from hasps at the door’s top and bottom, left and right. There’s a knob and keyhole on the left. As you examine the magical signs, the howling stops. The door shakes violently. An evil voice bellows “Release me!” Feeling a burning sensation you discover an ornate key on a chain around your neck.
UPDATE: This has been done on the spur of the moment. I have no plan. Links are welcome.