In Elden Ring, the Tarnished will come to find that Fia the Deathbed Companion holds ambitions to become the partner of Godwyn, the Prince of Death. This mission stands in stark opposition to the wishes of many at the Roundtable Hold, most of whom serve the Golden Order and despise Those Who Live In Death (the undead whose spirits have been rejected by the Erdtree). Fia thus represents an option for the Tarnished to act against the Grace that brought the two to the Lands Between, even if her questline does not make it explicit as to why that is.
Elden Ring's lore and storytelling works through a language of ambiguity and unspoken exposition: The Tarnished is expected to know nothing about the Lands Between and is consequently told very little. However, players can conduct research by reading item descriptions and considering where these items are found. Fia's role as a Deathbed Companion can be understood through this method by finding her dress beside Lionel the Lionhearted's corpse, or the Baldachin's Blessing. The Deathbed Companions come from a place beyond the Lands Between and serve Champions by laying with them, giving them blessings, and comforting them in death. In return, the Deathbed Companion absorbs some of the Champions' vigor with each embrace they share and then use this vigor to resurrect an exalted noble.
Fia's reason for associating with Godwyn is thus two-fold. First, Fia's goal as Deathbed Companion in Elden Ring is to comfort the dying, which is undermined by the Erdtree and the Golden Order, as having one's vigor transferred to a resurrected noble prevents their soul from being taken in by the Erdtree. This in turn curses them to eternal torment and an everlasting battle with hunters of Those Who Live In Death, which prevents the peaceful death Fia hopes to give. Second, to resurrect a noble, said noble must be dead. But because Marika removed the Death Rune from the Elden Ring, the Erdtree essentially grants people immortality by absorbing the "memory" of them, which makes the Deathbed Companion's mission pointless. Becoming the Deathbed Companion of Godwyn resolves both of these issues for Fia by granting her the opportunity to reestablish death in the Lands Between and in turn free Those Who Live In Death.
Godwyn's Second Life In Elden Ring
The resurrection of the Prince of Death is capable of bringing back death to Elden Ring and the Lands Between, because doing so requires creating a rune that can be used in the Elden Ring. In the story of the Shattering, pieces of the Death Rune - which Marika removed from the Elden Ring to grant people immortality - were used by the Black Knife Assassins to leave half of a curse mark called a Hallowbrand on two gods: Ranni and Godwyn. Doing so allowed for Ranni's soul to exist outside of her flesh, but this resulted in Godwyn's soul being stored in the curse mark separate from his body. Hence why in Fia's questline, the Tarnished must collect the pieces of the Hallowbrand to allow Fia to lay with Godwyn and resurrect him. The spawn birthed from Fia and the Prince of Death is the very rune that can be used on the Elden Ring at the end of the game: the Rune of the Death-Prince.
Thus, Fia's purpose to join Godwyn is to reintroduce mortality to the Lands Between. Though such a fate may sound macabre, it actually frees Those Who Live In Death who have been waiting to pass while also allowing Destined Death to reach the demigods who have become mad with age. In this way, death in Elden Ring is necessary for peace.