Elion is a god worshiped in many places in the world of Black Desert and his churches found in most larger towns like Velia and Heidel City. The symbol of Elion can be seen on the Ossuary of Velia and many flags.
He seems to be a rather benevolent god according to tales of his followers, who ask to be "filled with your light" etc. His symbol accordingly depicts flames and his color is red - the reason why his "holy servants", the Valkyrie, are expected to have red hair or dye their hair accordingly.
According to Elionians (?) Elion created lands, plants and animals by using the Black Stones. Afterwards he created the intelligent races and founded a city between earth and sky where overseers where located to watch over (and potentially control) the latter. Elion priests are actively out to mission followers and accept anyone willing to devote him or herself to their god. The Elion religion knows the ideas of heresy and sin as well as repentance. It demands that any sinner is warned three times about committing these - before they are abandoned. Elion is depicted as the savior of the world.
Interestingly there are slightly different versions of the Elion symbol in the game. While the Priests wear read and have the flames depicted with a T-shaped bar, the banners for Serendia show no flames and depict the symbol as a winged scepter on the Serendian colors of white and green.