Once you have enough of the story done, namely up through the "Potion of Blood" quest, then you will be told about the Children of Danu, an evil cult whose leaders you must eliminate. There are 10 total targets: 5 Lords, 4 High Druids, and The Oak.
As with the Order of the Ancients targets, you have to discover clues to find other clues that will allow you to pinpoint who and where your next target is. A couple clues/targets are related to other quests. If you take out the 7 that aren't before you advance the story, then you'll get #9 and 10 by the time you finish "A Scourging of Snakes".
All Children of Danu clues and locations, courtesy of GamerTweak.com
https://gamertweak.com/ac-valhalla-children-of-danu-location...