This guide for Assassin's Creed Valhalla: Wrath of the Druids helps you find the next of the Children of Danu, The Seed.
Where to find The Seed
If you took the time to kill the other members of the Children of Danu, then you'll be able to find him quite easily.
The Seed is hiding in the fortress at Dunseverick, northeast of Ulster. This is an area that is full of enemies, so if you don't want to go down the assassination route be prepared for a full-on assault.
By using Odin's Sight or your raven, you should find him quite easily.
This is part of a quest, The Wages of War, which you do late on in the Wrath of the Druids content.