Queen Marika the Eternal is the reigning divine sovereign of the Lands Between, and a vessel for the Elden Ring. Her offspring possess demigod status and are bearers of the Great Runes.

Marika's Rune can be seen in Marika's Scarseal and Marika's Soreseal and resembles the crowning arc of the Elden Ring, as well as her body crucified inside the Erdtree.
When Marika's body changes into Radagon, her dress becomes his skirt.
It is heavily implied that the Elden Beast was the being who imprisoned Marika inside the Erdtree based on one of its attacks where it attempts to grab the player and lift them up in the air to crucify them and then pierce them with a barrage of golden spears, similar to how Marika is seen crucified and pierced in the cutscene before the final boss battle.
Marika is reminiscent of the alchemical concept of the White Queen. The White Queen is often depicted as one half of the Rebis (the other half being the Red King, referenced with Radagon), a single being possessing both male and female qualities, also sometimes referred to as the divine hermaphrodite. It is said that the Rebis hailed from one being that is separated into a male and female half until it is unified once again to form a transcendental being empowered by a dragon (corresponding to the Elden Beast).
It is possible, but never confirmed in-game, that Melina is also a child of Marika. Her internal name in the game files is "MaricaOfDaughter", and most of the information about the Queen is gleaned from dialogue where Melina repeats Marika's own words at specific sites of grace.