This characters act alone for their own benefits. Some act as enemies while others make alliances or become allies with either Fairy Tail or anyone depending on their level of trust.

Acnologia: 

Absalom: 

Vulgrim: 

Samael: 

Spiritus: (demon OC made by Umbra74 from DeviantArt) 

Kronos: (another demon OC made by Umbra74 from DeviantArt)

Eligor: (fallen Angel OC made by Violyte64) 

Crowfather: 

Ostegoth: 

The Lord of the Hollows: 

Dis: 

Baul: 

Samuel Hayden: 

Novik: 

The Betrayer: 

Khan Maykr:

I'm not sure what I should do with the Dark Lord from Doom though, because in this story Mundus is the Demon Emperor and ruler of underworld. 

And the Dark Lord is supreme ruler of Hell in the Doom series. What do you guys think?