Kyanite
As they slowly but surely made their way down the long staircase through this underground structure, their expectations were forced to be shunned really.
It's not that they have nowhere else to go, but although they've spent the last half an hour walking and sliding down this Staircase, they've only counted two more passages, one of which is barricaded by time.
Feels like a small reward considering they've just overcome a 300 foot long staircase! Why would the Ignites build a staircase this big and build nothing else near it?
It is tricky to learn this place's history, almost impossible in some aspects but such acts occur only when something goes wrong. As if they were forced to abandon this specific work mid construction.
At the last few stairs there is more dirt, overwhelming this ancient architecture and at the very end of the Staircase there is no Stone wall, just dirt.