Dark Gods are a common archetype found in fantasy settings - though similar beings can be found in other genres as well, it is especially common in a "Rage Against the Heavens" plot in which the hero(es) of a story must fight against the power of an evil and/or uncaring god (sometimes even God himself (see God as Satan).
Since there were no true gods of "evil" in mythology, the usual candidates for the Dark God status above the Dark Lord status in modern tales are gods of the Underworld (mythology), divine immortal personifications of Darkness, or pagan deities of considerable fame - this can be confusing when looking at the older myths as many underworld deities were benevolent and/or amoral, it is theorized a lot of the negative traits of death-gods in the modern mind stems from the way death has become a symbol of evil while in the past it was (largely) considered a natural part of existence.
Trickster gods are also common candidates for this archetype and indeed in real-world mythology a number of trickster gods have shown traits that can be considered malevolent - such as Loki and some varieties of the Coyote.
Dark Gods tend to be portrayed as keeping their divine status and role (unlike demons) and even though many Dark Gods display traits and abilities associated with demons they are normally still in contact with other gods and have people who worship them: in some settings however the term "demon" can apply to fallen gods just as it traditionally applied to fallen angels.
Anti-Gods
The Anti-God, also known as an Anti-Deity or Anti-Supreme Being, is another varient of Dark God that represents the counterpart and sometimes the negative aspects of a Supreme God in many verses, they existed before the multiverse itself and are considered the essence of all evil and the ultimate decreator of the universe ranking them among the most powerful beings. Anti-Gods (known in plural as Anti-Deities or Anti-Supreme Beings) are beings of unlimited power giving them the ability to destroy entire universes; in some cases even compared to the supreme god, a clash between them could cause the universe to be constantly warped. These beings represent and are the prime originators of all that counters creation, giving it absolute domain over destructive powers.
The Anti-God from John Carpenter's Prince of Darkness.
Also note that not all Anti-Gods represent The Devil and some are known to be higher than Satan and may even be the parent of Satan (with the Anti-God from John Carpenter's Prince of Darkness being an example) or a being that is unrelated to Satan and yet acts as the true opposite of God. From a theological perspective, the Anti-God would be far superior to the Devil. As Satan, also known as Lucifer, is a fallen angel, thus a creation of God and by no means omnipotent. (Though very powerful). The Anti-God stands as a cosmic anomaly; an entity that is equal to God itself and thus far more powerful than any demon or any angel.