That key is really hard to see as it is in fire which is the same color as the key.
It's right at a T in the corridor on the lowest chamber I believe, right after you come in. To the left is the area with the sinking platforms that drove me so nuts I broke a mouse by slamming it down and nearly put my fist thru my monitor.
I got junk out of that chest by the way, just some potions. It's random though so may get something good.
