Deathbringer is an unlockable character within Call of Duty: WWIINazi Zombies, he can be unlocked by completing a full challenge set in the map The Frozen Dawn, in this case being the "Desert Assassin" challenge set.
Challenges
Some things are best... - The Blood does not flow (Left forgotten)
Get to Wave 15 without opening any door.
Bloodied Sacrifice - Blood rains from the heavens, pools at your feet (Guarded Sacrifice)
Kill 10 zombies while they are in the blood pool in Blood Altar by charging them with the Roar of Sang'ket.
Zombie Torero - Precision over force (Needless effort)
Using the Thulian Transports, transport the Wüstling carrying the Thulian Shield from Blood Altar to Crash Site and from Crash Site to Passage. Only the Wüstling has to use the transport.
Stalwart Soldier - Nothing can shake you (Relentless)
Let the zombies damage the Thulian Shield enough times without breaking it.
Blood is power - Descend into the darkness and raise again, greater (Light in darkness)