The Last Holdouts is received if you sided with Meredith during the Showdown conversation at the start of Act 3 and made already some effort on the quest with the same name.

Letter Edit

My men report that the blood mage hideout was cleared of all maleficarum. The Maker has blessed you with formidable strength, and it is fitting you use it to aid our righteous cause. We uncovered the location of their headquarters: a cave in Sundermount. Ser Agatha and I are gathering the faithful and will await you there.

-Ser Mettin