If it makes you feel better, I needed to look up how to enter the temple of death myself.
It would have never occured to me to send down one of the pyramids, since I don't consider these items 'dead', but rather inanimate.
The mirror you're looking for is present on the surface of the Luculla map. There are two mirrors present. One is the exit mirror from Loic's (or whatever his name was) basement (the building in Silverglen, where you took the Immaculate's first test). Needless to say, you'll be needing the other one.
