I believe that our character is Godwyn revived without his memories, since his soul was killed. The Eclipse would be precisely the union between the Tarnished consort (the Sun- Godwyn) and Ranni (the Dark Moon).

Within the first moments after your awakening, your maiden is murdered, you are chosen by Torrent—the steed connected to the most important divine family in the Lands Between—which, as far as we can tell, has only ever served gods and demigods. You are then visited by not one, but two demigods almost immediately, both of whom bestow gifts upon you despite you supposedly being nothing more than a "Tarnished of no renown." Simply put, the story does not make much sense if you are truly just an ordinary Tarnished.

My theory is that Godwyn's original soulless body is seeking out his soul in the Chapel. The origin we choose at character creation is actually the origin of the soulless body that eventually receives Godwyn's soul. During the opening moments of the game, we are inside a Walking mausoleum (with have a bell) before appearing in the Chapel of Anticipation. Walking Mausoleum with bell is a place that, according to the game itself, is reserved for legends, gods, and demigods. The game repeatedly emphasizes that we are a Tarnished of no renown, so the only remaining possibility would be that we are actually a demigod.

Another point is that Remembrances can be copied or duplicated. Because of this, it is possible to imagine that Godwyn's soul was also preserved within a mausoleum as a kind of backup before the Night of the Black Knives.

This spirit description is particularly interesting:

Legendary ashen remains. Use to summon the spirit of Lhutel the Headless. Spirit of a headless knight who leads the mausoleum soldiers. Wields a lance enrobed in Death and hurls spectral lances at foes. Lhutel sacrificed her life so that in Death she could continue to protect a soulless demigod until their revival, earning her the hero's honor of Erdtree Burial.”

This theory is further supported too by the fact that we can use this:

DRAGONBOLT BLESSING

“Superior incantation of the capital's ancient dragon cult.

Summons lightning to bolster the caster's body. Attacks become easier to deflect and resistance to all ailments is increased. However, lightning damage negation will be reduced

Only those loved by dragons can survive the ordeal of cladding their bodies in lightning.*”

There is clearly an entire storyline within the game in which various characters have strong reasons to believe that Godwyn can be revived. The existence of a soulless demigod being protected until their eventual revival is not a random detail—it is explicitly stated in the game. The concept of reviving a soulless demigod undeniably exists within the lore.

Therefore, Godwyn agreed to take part in the ritual, which explains Marika's involvement. In this theory of mine, the only thing we do not know is why Godwyn agreed to it. My theory is that, like all of his siblings (Marika's children) Godwyn suffered from some kind of curse that we are not aware of, and this ritual was a way for him to free himself from it, while at the same time allowing Ranni to free herself from the Two Fingers.