Lore summary
The Greater Will is an alien that made Marika into its avatar, using her to wage war against the various factions already existing upon the planet to further spread the influence of the Erdtree. The larger the territory of the Erdtree, the more people are marked by it. When these marked people die, the Erdtree absorbs their souls and allows those it deems useful to be reborn. Marika realised that the Erdtree is a sinister creature that feeds off those it deems unworthy and only spares those that can further it's influence in order to gain more territory/souls. The Erdtree was reaching the end of its lifespan, whereupon it would suck out all life from the Lands Between and once again spread out it's seeds throughout the galaxy to repeat the process. This is why she first tampered with the Elden Ring and took out the rune/concept of Death. By removing Death, the Erdtree's growth was stunted, there were less dead for it to feed upon. This gave her time to send away Godfrey and have more children that would help her wage war against The Greater Will. She encouraged Godfrey and his men to go out and wage war in order to harden themselves and give herself a standing army outside of the Greater Will's influence. At the same time she encouraged her followers to question the Golden Order, in hopes this would lessen the Greater Will's influence upon the people in the Lands Between.
She succeeded at having a child that would defy the Greater Will, however it backfired spectacularly. Ranni stole a FRAGMENT of the Rune of Death and used it to kill her body in order to break the influence the Greater Will and Two Fingers had upon her. The weapon she used to kill her body was taken away by Assassins in the service of the Two Fingers and were used to kill Godwyn in order to begin the process of mending the Elden Ring and returning Death to the Lands Between. They turned Marika's most splendid child, who had befriended dragons and was unsullied by other Outer Gods, into a soulless puppet. An eternal living manifestation of death, that would spread it among the Lands Between once more. This drove Marika to the brink; she lost her means of stalling the Erdtree for time. Since the Greater Will practically declared war upon her, she fought against Radagon (She had cast away the parts of herself most susceptible to Outer God influence in the same way Ranni destroyed her Empyrean body) and shattered the Elden Ring. The Erdtree sensed the danger this act posed and shot out countless seeds to act as their own failsafe should the main Erdtree fall. Marika was neutralised and sealed away by the Greater Will. While sealed away, all she could do was attempt to guide her children, who were unfortunately corrupted by other Gods and could not fulfill her wishes to break free of the Greater Will. So she turned her gaze to the kin of Godfrey, whom she had sent out to become her secret army against the Greater Will.