Well, Shit

Seeing Red is a companion quest for Varric Tethras in Dragon Age: Inquisition.

"Bianca says Corypheus is getting his red lyrium from the thaig Hawke and Varric discovered years ago - on their expedition into the Deep Roads under Kirkwall. It may be possible to reach it using an entrance to the Deep Roads somewhere in the Hinterlands."

Acquisition
Available after completing the main quest Here Lies the Abyss.