The spirit form of Baron Umbra.
Baron Umbra was the mastermind behind the Nazi occult forces as well as the one who helped in forming Plan Z with the Nazis. He was also responsible for guiding Dr. Efram Schweiger into making the Z5 cores as well as the RacheGeist. Umbra was the reason why Schweiger turned on his allies, wanting to let them and himself use the occult energy to his and their benefit.
He is the overarching antagonist of the DLC maps of Zombie Army 4: Dead War and the main antagonist in the Return to Hell DLC map. Baron Umbra was also the steward of the ancient castle in Transylvania that was seen in the DLC map Blood Count. He was finally destroyed in "Return to Hell" by Schweiger after the Survivor Brigade squad disrupted the baron's ritual.
With Baron Umbra destroyed, the cult was forced to retreat deeper into Hell where they began to unleash Project Ragnarok from a hidden stronghold.