Children of the Old Ones is a Lizardmen faction introduced in Total War: Warhammer II. The faction is special since it spawns when Markus Wulfhart of The Huntmarshal's Expedition Hostility level reaches 5. This Lizardmen faction will try to reclaim Lustrian lands from the invaders.
All of the units led by the Children of the Old Ones are Unbreakable, meaning they will never rout.
You cannot interact with this faction diplomatically.
Seeing as all of its units are unbreakable and Lizardmen units excel in melee, it is best to play to the strengths of the Empire and The Huntsmarshal Expedition. This mean extensively using missiles and artillery to fell your foe before they can reach you, or using a War Wagon to distract certain units away from the front line engagement.