Skullhell

Skullhell is the god of darkness and Lumiaris's nemesis, the 2 of them having been waging war on each other since time immemorial. However, according to Darunggar's Legends the 2 use to be one as Enos (literally: one god), but one day that god split into 2 conflicting half.

Appearance
On a tablets found in Darrungar, where Skullhell and Lumiaris once fought, he is depicted as a man with a skull-like face and missing a leg. For weapon he shown holding a large scythe in his right hand and a sword in his left. He also likely wear some form of clothing or armor as in the same description it's mentioned that Lumiaris is naked, implying that Skullhell isn't.

Personality
Unknown, but if he was the one behind Zodiac sudden evolution he may care more about his followers than one could expect a Dark god to.

Darunggar's legend
According to Darunggar's legends, Lumiaris and Skullhell were once one and the same as Enos (one god). At that time Darunggar was called Fanangar (paradise), but after Enos split, the 2 nemesis gods gathered massive massive armies to wedges wars on each other. Eventually, the whole world was at war and became Jidgar (field of battle). Finally, the battle between the god came to an end, how or why now lost to time, but both gods left for Grimgar leaving behind many of their followers in a now-destroyed world that finally took its current name: Darunggar (the land of despair), a godless world.

Considering the state of Darunggar, the fact that that legend seems to be widely known, the way divine magic works there, and the fact that there's some archeological evidence to back it up; it seems very likely that that legend is mostly true.

Grimgar's legend
In the myth of Arabankia, a story told since ancient times about a paradise-like land hidden somewhere in the world and a founding myth for (at least) the Kingdom of Arabakia, it's said that Grimgar was once laid waste in the terrifying battle of the gods.

Unfortunately, no more is said about that war, so we don't know who the waring gods were, if they were native gods or not, how fierce the battle was, or how it stopped. Moreover, we're explicitly told that that story is just a legend, likely based on some truth but heavily modified, with many versions, and that it was even used for political means at least one time.

As such, that story of a great war between the gods could have originated from people fleeing the war-torn Darunggar, but over time, shift to take place in Grimgar as most people probably wouldn't know of Darunggar.

That said there's still the possibility that for a time after they fled Darunggar, Lumiaris and Skullhell restarted their war, in Grimgar this time, before stopping for some unknown reason.

Level. 11 At That Time, We Each Dreamed on Our Own Paths
If what Zodiac says to Ranta is to be trusted, every demon is a part of Skullhell. While that part is doubtful considering Zodiac tendency to mess with Ranta (Ranta considering that it was just a comedic bit), Zodiac sudden evolution despite Ranta having been unable to offer vices for a long time does seem to be indicative of some intervention by Skullhell.

Abilities
While we don't know the true extent of his abilities, we do know that he is able to give every dark magic user access to a small amount of his power.

And according to Darrungar's legend, his battle with Lumiaris made the whole world a battleground, before finally breaking it, leaving that world in the state we see in the series.

Finally, it seems like he can see through the demons Dread Knight summon and affect their evolutions.

Lumiaris
The 2 use to be one god named Enos which split into them long ago, in Darunggar. As god of opposite element (Darkness and Light) they seems to have been in conflict since the beginning, their war fierce enough to lay waste to world(s).

By the time of the story they're not waging open war anymore, and even allow volunteer soldier from both faith to work together. However, the Church of Lumiaris seems to be the official religion of the Kingdom of Arabakia and from what Neal said in Level.15, the cult of Skullhell isn't allow in the mainland.