Use the Mother's Flame on Obsidian Eggs to release whelps. Follow those hatchlings to the lair of their mother.
In the southwestern portion of the Obsidian Forest, you will find some of the black egg clutches. Many of the eggs can be hatched at any time with a bit of prodding from the right flame, and as the whelplings hatch, they will cry out and flock for their mother.
Do not worry, if this purging is to be taken to the whelps, that is not a task that I will force on you. I simply want you to let them out and follow them home as they seek their mother. It is she who we must confront.