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Book 3 theory on Eir
Okay boyoes, I got something for yall.
EIR IS DEAD SHARENA
Firstly lets set up a timeline. The first event is Sharena's death at the start of book 3. This comes first in the timeline since it happens in the cut-scene that plays before we start Book 3. We don't meet Eir until we start book 3, when Sharena is already dead.
So from a timeline perspective, Eir being Sharena COULD happen.
Secondly, Feh doesn't say much about Eir, but she does say some interesting things about her; such as that Eir is a princess, just like Sharena.
This also solves two questions that no one is asking;
1. Why does Eir join us when she is the princess of hell?
Because if she is actually dead Sharena than of course she would join us.
2. Why is Eir, princess of death a light attribute?
Because she is actually Sharena, the light of the order of heroes.
Also her face looks the exact same, but that could be circumstantial.
Tell me what yall think down below.

Fate Grand Order
Dragonball Legends
IV Calc
Heroes