Personally, I think the Jailer is Kel’Thuzad. The most powerful Necromancer to ever Necromance. The dude who made the Dead Scar in Quel’thalas. He who we never truly got rid of because we never destroyed his magic vase of resurrection.
If anyone has the power to ascend to God-tier level in the Shadowlands and start funnels souls, it would be Kel’Thuzad.
Also, the outline of the Jailer we got just screams Lich to me for some reason. Maybe he was sent to the Maw in WotLK and from there took over the role from the previous Jailer, growing more and more in power over time until he started funneling and hijacking souls from the other realms?