The final room you're looking for is actually on the upper level of the Temple and is called Bliss. After Desolation there are two more insignificant rooms and then a square-shaped stairway which leads to the first room of the upper level known as Ignorance. Then on to Bliss. 
After the final room the group will travel back down the stairway. I'll have my secret passage come in some time around then. ^_^
--------------------------------------------------------------
Riordan’s attention does not immediately return to George, his eyes focused on the angelic statue in the center of the room. Chills run down his spine. This place... it had such an eerie feel to it; such a haunting. The noble felt as if everywhere he went he was being watched by unseen eyes and that put him on edge. He looks up at George and gives a simple nod in agreement, walking to stand before the massive doors which block the path to the next room.
With a deep breath he reaches out and touches the doors, power surging in the air around him, the simple action causing the stone to groan as it slowly moves from its resting place. With such ancient foundations being disturbed, dust rises into the air around him and the others, clouding their view of the next room. However, George manages to catch the ghostly figure of a woman running away in the darkness before she disappears completely.
Riordan reaches into his robes and pulls out Shouten Yaiba’s grip, his thumb trailing over the ripples in the design carefully as the air around them clears revealing the dark and ruined vastness before them. The noble is shocked by what greets his gaze: crumbling pillars, tilted as if their very foundations had been shaken; piles or rubble and stone almost completely covering the ground as if a god of destruction itself had walked through this Temple.
His blood runs cold. Something had happened here... no simple curse could cause such devastation. Mortis... had unleashed something. He glances back over his shoulder at the others. “Ignite your blades so that they may light your path. Evils are fond of such desolate and darkened places as this... but are often cowards and will not reveal themselves in the light...”
Riordan points Shouten Yaiba’s grip towards the ground and closes his eyes. “Ashes rise now and become my shield...” he whispers darkly, his voice deep and low. “Flames burn now and become my blade... Shouten Yaiba.”
With his last words Riordan brings his arm up and then extends it as if cracking a whip, a dark voice -not belonging to Riordan or any others present- whispering through the Force. “Shouten Yaiba...”
Flames suddenly consume the grip of the sword, climbing up the noble’s arm until they are violently sucked back down to the grip and erupt in a massive fire whip where the sword’s blade once would have been, the weapon dancing around Riordan’s body and bending to his every whim. The noble grasps the split end of the whip with his free hand, the flames wrapping around his arm like a serpent.
He looks back at George and the others once more, studying each for their reactions.
---------------------------------------------------------
Ah, our first vampire release! ^_^ So exciting! In order to to keep it from completely crushing them, vampire’s (or those yet to turn) can share their power with a weapon, giving this weapon a unique personality and appearance depending the vampire’s powers. The weapon normally appears as a simple object (i.e. Shouten Yaiba as a katana), however, when it’s master -and only its master- utters a unique release command it takes on its true form (i.e. Shouten Yaiba as a flame whip).