This is Justicestar, who was the host of (not possessed, that's different) a dark spirit. She was soon consumed by the dark spirit and became their leader, and gained her dark markings, the symbol of a dark spirit. (Her original form was Skyfern, and that's a different story.) She took over the darkness, possessing every warrior, making them follow her every command, and to destroy the light, whose leader was new and inexperienced. Justicestar had many battles with the light leader, but each time the light leader (whose name was Leo, by the way) was defeated, she took a new form, a new host, destroyed them for the good of their Clan. Justicestar always won. Everything lost balance. But Leo never gave up and never gave in. They were both tired of fighting. But they had to. One boring day, Leo won the battle. And... she won the war? At least she released Skyfern's spirit, the greatest Light warrior there was. Skyfern convinced more light spirits to join them in the battle against the darkness. But Skyfern's only remaining kit, Windwhip, was part of the darkness. And she was doing a good job of getting more warriors for them, too. Skyfern's first kit, Icestar, didn't participate in the battle; she sacrificed herself for a prophecy and prophecy cats weren't to fight; and her other kit had done the same. But then another boring day, Justicestar made an announcement. The light was just as much as evil as the darkness. They were taking random cats from StarClan and other random spirit-cats. They were fighting only to win. The light and darkness were one once again; or were they? Justicestar's spirit still remained...
Credit to @NegativeNarwhalz for the lineart and coding. I forgot where I got the backdrop from. The OC is Justicestar, who belongs to @EVB379968 as well.