The Ash is the second of the Children of Danu that you will encounter in Assassin's Creed Valhalla: Wrath of the Druids.
Once The Wren is taken care of, you'll be given the option of hunting down The Ash.
The Ash - First Clue
To discover the identity of The Ash, you'll first have to find its location. Investigate the Druidic Ritual Site found between Lough Tuam and Lough Rea.
The Ash - Second Clue
Next, head to Kiltober. You'll find a note inside a small ten on the southern side of the camp.
The Ash - Third Clue
Next, head to the Kesh Corann mines. You'll find these northwest of Lough Gara
The right path is the one through the highest cave entrance. Be prepared to take on a lot of enemies in this area, and watch out for poison.