Elden Ring is centered around the Great Will and, accordingly, Elden Beast. As an External God, the Great Will has a hierarchy of servants who help him in his plots to gain and maintain power over the Intermediate Lands. While Two Fingers play a big role in direct communication and transmission of the will of the Supreme Will, the Ancient Beast plays an even more important role.
[Attention: this article contains spoilers for the Elden Ring.]
Elden Ring takes place in the Midlands, a place ruled by Queen Marika and her Golden Order. When the Tarnished Champion returns from his exile in the Interlands, Queen Marika has already been imprisoned in Airdtree for destroying the Old Ring. Tarnished follows Grace to the Midlands to find a way to repair the Elden Ring and either restore the Golden Order or open a new era.
Elden Beast is not a mandatory and final boss in Elden Ring. It is said that the Ancient Beast was contained inside a star that the Great Will sent to the Intermediate Lands, and then it became an Ancient Ring. When Marika was chosen to rise to divinity and become a vessel of the Ancient Ring, she had to become one with the Ancient Beast to become a vessel of the Ancient Ring.
Elden Beast — vassal of the Great Will in the Elden Ring
Instead of going to the Lands Among themselves, the Great Will sent a vassal and ambassadors instead. A vassal rules a piece of land instead of the owner of the land, so the Ancient Beast acts as a proxy of the Great Will in the Intermediate Lands to fulfill his wishes, making him the embodiment of order. The strongest sorcery of the Old Ring comes from the Great Will, and this is due to the influence of the Great Will, spread because of the Oldest Beast and the Two-Fingered. Since he transforms into an Ancient Ring and then transforms back into a beast to fight when the vessel is destroyed, the Ancient Beast is primarily an extension of the Great Will.
The intervention of the Higher Will in the Intermediate Lands is perceived either as a blessing or as a curse, depending on who is asked. Although the Ancient Beast, who brought the Ancient Ring and Mercy to the Intermediate Lands, opened the golden age, but it did not last long, and Marika, who destroyed the Ancient Ring, caused a destructive war in the Intermediate Lands, when the demigods fought for power. Despite their influence in the Midlands, very little is known about the Outer Gods and those who embody their will, such as the Ancient Beast in the Old Ring.