This quest begins immediately after your conversation with Damiane at the end of the "Exposed Plot" quest. You can now read the letter that was previously sealed.
The power sealing the letter from White Crow dissipated. Read the unsealed letter.
Reward: Gear Blueprint: Haste
Open your inventory and read the Unsealed Letter. Once you do, you will receive the Haste gear blueprint and the materials used in it. A notification will also appear stating that completing faction quests is sometimes necessary to progress the main story.
The faction quest in question is "The Witch of Wisdom". It will be automatically added to your journal and you can find it under the Faction Quests > Hernand > Witches tab.
Its first objective is "Examine the house". To do this, you must travel to Meshley Trapsmith. There, you will encounter several bandits that you'll need to kill.
Next, enter the area - watch out for traps - and climb through the open roof window into one of the houses.
Inside, you will find a tied-up woman named Tanya. Free her from her bonds and she will reveal herself as a witch. She will invite you to her home and then vanish.
The witch lives nearby, slightly to the northwest - Witch's House. Head there, enter the house, and speak with her by selecting the "Listen to Request" option.
At this point, this quest will end and you will receive the next one - Crowcaller.
Elowen, the Witch of Wisdom, spoke of the coven that safeguards the delicate balance between the Abyss and reality. She offered a grave warning of a malevolent force on the rise that seeks to neutralize the witches, seize their sanctuaries, and plunge the world into darkness.