Death

The Shadow, as it is referred to by The Disciples, is the leader of all demons. It is the natural force of Death with a conciseness.

Background
The Shadow worked in secrecy foe hundreds of years, before revealing itself to numerous demon masters, including Lord Loss. It overpowered them and became the leader of all Demonata. Because of its power, it promised all demons that they would have eternal life. It also brought back the dead in the form of zombies. "(Death's Shadow")

Putting its plan into action
The Shadow worked with Lord Loss and Nadia Moore to achieve his goal.

Alternate Timeline
Note: This future has been prevented by The Disciples

In the alternate future seen by Grubbs Grady, Kernel Fleck, Bec McConn and Beranabus, The Shadow's army had invaded all of Ireland and were on their way to destroying the Earth. ("Demon Apocalypse")

Appearence
The Shadow looks like a giant octopus, with many tentacles. There are mouth's on the tentacles. It has very good grip, as Bec couldn't escape from it without assistance. ("Death's Shadow")

Power and Abilities
The Shadow is Death, so it has the power to resurrect the dead. It resurrected Nadia Moore and thousands of people on the ship where it fought Beranabus.

It can also heal itself from all wounds, as revealed when Beranabus stabbed it.

Trivia

 * The Shadow is the only known demon more powerful than Lord Loss.