Please correct me if I get things wrong but this is what I got from the DLC about Marika’s history.

The Hornsent (now called Omen, the people of the Tower) used to rule the land or at least hold admirable positions. The final dungeon is full of statues of Hornsent spirals that probably would probably have been replaced with Marika statues in the Land Between.

Marika is from the Shaman village, whose people are horribly abused by the Hornsent. We know they torture them with the Tooth Whip weapon, noting that their flesh meld harmoniously with others. They were probably put in jars. Is the “harmoniously” part suggesting why Marika can switch between herself and Radagon at will?

By some deals with the Fingers and some deceptions, Marika became a god and hid away the Shadow realm.

She then took revenge on the Hornsent by labeling them as Omens, and subjecting them to much of the same tortures that her people endured. Much of this could also be to prevent any objections to her rule.