Apothicon

"The most powerful and evil entities in all the existences that ever... existed. And now they're here in our perfect world."

- Dr. Monty The Apothicons are a group of Keepers that were corrupted by the dark aether, with their only goal of wrecking havoc across all of creation. The Shadow Man is a known Apothicon. Many weapons, such as the Apothicon Servant and Apothicon Swords are named after them.

Summary
The Apothicons were originally part of the Ancient Order of the Keepers, but upon contact with the energy of the dark Aether, part of the Keepers were corrupted and betrayed their brethren. After a war between the two forces, the corrupted Keepers were eventually defeated, and what's left of them was banished into the dark Aether beneath creation for all eternity.

After spending time trapped in the dark Aether, the corrupted Ancients morphed into hideous creatures that shared little resemblance to their former species. During the Middle Ages, the Apothicons managed to escape their prison and invaded Earth, but were fended off again by the Keepers, this time with the help of human knights and were sealed once again within the dark Aether

Some time later, in Shadows of Evil, an emissary of the Apothicon overlords managed to trick four cursed souls into releasing the Apothicons in their dimension using the Summoning Key, but the creatures were stopped with the help of the Keepers, with the Shadow Man trapped inside the Key.

In the Revelations opening cutscene, Maxis gets manipulated by the Shadow Man within the Summoning Key and frees the evil Apothicon, who in turn summons the rest of the creatures.