The Pleiostoic Era Shadowrealm is a Shadowrealm created by Marethyu and his all powerful Aether Hook. The Shadowrealm was modeled on the time Marethyu was born, before he was taken away Isis and Osiris and transported into another Shadowrealm.
This Shadowrealm has all the creatures that would have resided in that time, including sabertooth tigers and wolves. This world, since made to Marethyu's desires, has no snakes and or creatures that barely resemble snakes. This Shadowrealm also has two coloured moons, a night sky that closely resembles the Milky Way Galxy, and a large lake in the middle of the Shadowrealm. Which, if he desires, can change flavor or the properties of that lake.
Marethyu brings Joan of Arc, Scathach, Comte de Saint-Germain, Palamedes, and William Shakespeare to the Shadowrealm by force to explain to them that he needs them to bring about the destruction of Danu Talis. Joan and Scatach get transported there first, after the ley gate at Notre Dame was tampered with, sending the girls back in time. They fend of the local wildlife, and then meet up with the others, who have been granted to go to the Shadowrealm by Palamedes's Elder master, Tammuz, settling an old debt. Marethyu soon after leads the group through a series of complicated ley gates from Traitor's Gate to the Isle of Danu Tails before it sank. Marethyu then dismisses the Shadowrealm, no longer needing it, so it crumbles to dust as soon as he leaves.
- Marethyu probably modeled the night sky off the Milky Way Galaxy because his former 'parents' Richard Newman and Sara Newman had painted the Milky Way nightsky on the ceiling of his sister, Sophie Newman's room at their old house. This probably brought back good old memories of the time when things were simplier.