Flame House Key
From WikiRaider
The Flame House Key is a stone artefact found in Tomb Raider Underworld, in the Mexico level The Midgard Serpent.
It is found in a side chamber of the court with the statues of the Lords of Xibalba who guard the entrance to the underworld. To obtain this key Lara has to overcome a set of fire emitters. She then needs to insert it into one of the four slots on the table in the court of the Lords of Xibalba. This key has been decorated with red colour.
A token considered to be a gift to the Lords of Death, and only obtained through trial by ordeal.
Description in the PDA in Tomb Raider Underworld