Chronicles, and Xal'atah for that matter, state that the Valarjar army in the Halls of Valor are locked in there by Helya. It is the strongest army in the world and it is useless because it is locked.
However, as a warrior player you can send Valarjar troops and champions on missions across the Broken Isles.
What. Is going. On.
How can those guys leave the Skyhold?
Halp!