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Stalgard says the passages leading to the Titan's heart are now peaceful and no longer lined with red lyrium. The jagged crystals we saw retreated back into the rock, the veins are blue again, and sing like lyrium usually does.
It makes some sense, I think. The disembodied rage of the Titans, with nowhere else to go, took root in the heart. And the red lyrium extended from there, her pain infecting what was left of the Titan's body. When I accepted that rage into myself, it cleansed the mountain.
The red lyrium we saw in the heart looks like blighted lyrium, but it's not. Blighted lyrium is almost impossible to destroy. It infects and grows and corrupts, just like the blight itself. What we saw in the heart wasn't it.
Kal-Sharok's scholars don't have an answer. I wonder if I could introduce them to Dagna. She'll have theories I'm sure.