I’d soooort of imagined it as a fight against Achroma before because she’s the mascot of Eternity but

What if Aeterna’s fight against time won long in the past was actually a fight against Era, who wanted to siphon up Aeterna just like he did to the Spanish lion kingdom and the other places, but nope Aeterna beat him

Possibly happening against a future Era (I don’t see “present” Era as doing this, so some thing the main characters do might have the possibility to make him do this) that went and attacked past Aeterna for some reason, maybe thinking they’d be easier to overcome because they’d be divided and not knowing that defeating him was what would unite them because he hasn’t seen that part of his timeline yet