Why did Ranni steal the rune of death?
In order to escape her fate, Ranni plotted to slay her own empyrean flesh while sparing her soul. In order to do so, she stole a fragment of the Rune of Death from Maliketh, the Black Blade.
Desiring a world without Destined Death, Marika took the Rune of Death from the Elden Ring and charged Maliketh with protecting it. Maliketh's blade was imbued with the power of the rune, making him a peerless foe for any who would challenge Marika's order.
The reason Ranni had Godwyn killed was for a ritual. She wanted to destroy her body but preserve her soul in a doll. This way, she could escape her destiny and not be bound by the whims of the Greater Will like the rest of her family. It was this act that really made the plot of Elden Ring begin.
A perfectly orchestrated plan to slay a Demigod, perfectly timed so that she could ascend to her soul form. Ranni wanted to overthrow Marika, and to do that she would need the power to kill a god. A power that only existed in the Rune of Death.
It's soon revealed that the Rune of Death was stolen by Ranni herself, taken from Marika's bodyguard and keeper of the rune, Maliketh the Black Blade. Ranni was then able to create knives powerful enough to kill immortals, which were used on Godwyn the Golden.
Radagon is the male half of Queen Marika and former husband of Queen Rennala. As Marika's King Consort, he became the second Elden Lord.
The Ring was broken by Queen Marika the Eternal after the death of her son Godwyn. By destroying the Elden Ring, which acted as a connection to both the Greater Will and the Golden Order, the Greater Will's influence on the Lands Between was greatly diminished.
Ranni wished to shed her empyrean body, but preserve her spirit, which meant that someone else's spirit would need to die; Godwyn's. The assassins fell upon the Royal Capital and murdered Godwyn, his soul destroyed by Ranni's ritual leaving only his soulless body behind.
Lunar Princess Ranni assassinated Godwyn with the help of her Black Knife Assassins, causing Queen Marika to shatter the Elden Ring in her distress. However, striking down a Demigod isn't easy, which is why Ranni imbued the Assassins' Daggers with the Rune of Death, stolen from Maliketh.
Zullie theorizes that Godwyn's cursed death can be associated with the Ningyo, a mythological Japanese creature. The Ningyo is akin to a mermaid, often drawn with a fish's tail, clawed and webbed fingers, seaweed-like hair, and fishy features.
Why did Marika banish Godfrey?
But after his last worthy opponent fell, Godfrey's warrior spirit sank, the hue from his eyes faded and he lost the guidance of grace. His purpose now fulfilled, Marika exiled Godfrey and his warriors from the Lands Between, making him the first Tarnished.
Players are originally led to believe that Ranni orchestrates this to kill her body, which is true, but that the death of Godwyn so upsets Marika that she destroys the Elden Ring out of emotion.

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She lifted her hammer and shattered the Elden Ring; while Radagon simultaneously tried, and failed, to repair it. Unable to remove Marika as its vessel, the Greater Will imprisoned her in the Erdtree along with Radagon, the two bound together in a single body.
They're a lot like the Chosen Undead and the Hollows in Dark Souls. They're cursed to immortality, bound by a singular goal, and driven to a choice, working tirelessly to fix an apocalypse wrought by the gods who spurn them.
Why am I not getting all my Runes back in Elden Ring? The Gatekeeper steals part of your Runes each time you die in Elden Ring. That's why you don't get all of your Runes back after you die. To get them back, you need to kill the Gatekeeper before you kill Godrick.
Morgott is the true identity of Margit, the Fell Omen, and the self-proclaimed "Last of All Kings". He is found in Leyndell, Royal Capital. When he and his twin brother Mohg were born, they were imprisoned deep in the Subterranean Shunning-Grounds for being born as Omen royalty.
The clue left outside the Statue of Radagon hints at this word - Regression - and simply casting the spell in front of the Statue reveals its secret. Unlike most Incantations, Law of Regression is based on Int rather than Faith as it is a Golden Order Incantation.
The Elden Beast is a vassal of the Greater Will and the living embodiment of order itself. The Elden Beast came to the Lands Between as a golden star sent by the Greater Will, and became the Elden Ring. It became one with Queen Marika the Eternal, a Numen Empyrean chosen by the Greater Will.
Stakes of Marika are the markers for your character to respawn after you are eliminated near any Stake of Marika; you will be respawned at the same location that you died in but at a safer site.
What is the Rune of Death?
The “death rune” is also known as a variation of the Algiz or “life rune.” Origin: This rune is the fifteenth letter of the Futhark runic alphabet, an ancient Germanic script mostly found in Scandinavia and the British Isles (in another version of the Futhark alphabet, it appears as the sixteenth rune).
Elden Ring: Marika's Plan to Destroy the Elden Ring and Kill the Greater Will. Regarding Elden Ring's Rune of Death and Fire of Ruin, it's possible that Marika did these with an alternative motive—if players buy that she was not as impulsive as the original story suggests.
Stakes of Marika in Elden Ring are a temporary respawn point in the form of a Statues. You will first need to interact with one of these statues; they are generally found near important landmarks or boss locations. After interacting with one, if you happen to die nearby to one, you'll have an option to respawn there.