Ok, I've gotten to the final area of the fifth level, the "Temple of Darkness..

Question for The Legend of Zelda: Four Swords Adventures

Ok, I've gotten to the final area of the fifth level, the "Temple of Darkness", and I need some help. In the room where there are several stalfos heads laying around and on the middle (where you have to light four torches in the middle so that the stalfos head in the middle will dissapear and leave a hole), I lit the 4 torches, fell in the hole, which plunges you into pitch blackness. Except for the chests and stairs. In one of the chests there is a key. Now, this is where I'm stuck.

I have yet to find anywhere to use this key. The only door or passageway or whatever that I haven't been into yet is a closed door on the north side of the room located one room east and one room north of the room with the several stalfos heads. This door, however, can't be opened with a key. I assume it opens when a certain event is triggered, but I'm not sure what event I have to perform.

Perhaps I need to kill all the monsters in the room, but the problem with that is that there are two anti-fairies in the room, and I don't know how to kill them. (in other games, you use the boomerang to kill them, but that doesnt work for this game) Anyways, hope someone out there knows what I'm talking about and can help!