Shez and Byleth fight, and Shez gains the upper hand using her Arval power. She's just about to strike the killing blow when bam! Divine pulse activates and suddenly Sothis has possessed Byleth, green hair and all.
Sothis and her bullshit time powers let her easily gain the upper hand, teleporting all over the place and basically playing with Shez. She's trash talking Shez all the while and seems to take her existence as a personal affront. Only Byleth fighting back and resisting Sothis' control allows Shez to escape.
The scenes afterwards on both sides are very illuminating. Shez and Arval are once again licking their wounds and talking about the battle. Noticably Shez is very disheartened due to the perceived power gap and it's only due to Arval's pep talk that she doesn't give up. Despite her rivalry with Byleth she's not willing to throw her life away and is even open to the idea of Edelgard paying Byleth off to come over to their side. Arval is vehemently against even the idea of teaming up with the Ashen Demon and when Shez asks why they mostly evade the question and prey on her desire for revenge to convince her. They also seem to have some awareness of Byleth's connection to Sothis.
On the other side Byleth and Sothis are having their first proper meeting. Sothis berates her for preventing her from killing Shez, which someone familiar with her character from Three Houses would expect. Contrasting their meeting in that game right off the bat she seems to have much less respect for Byleth and her personhood, comparing her to a glove disobeying the hand. She also seems to have more of her memories as the mother goddess to Fodlan?
Anyway it's pretty clear both Arval and Sothis have at least some idea of the other's current existence and connection to their vessel/host. They seem hellbent on removing each other from existence as well as they can as well, to the degree that they don't stand for Shez and Byleth having some degree of mutual respect and willingness to live in peace. My guess is that Arval was a deity native to Fodlan who came into conflict with Sothis when she first arrived from elsewhere (space!), and has some connection to the Agarthans though I'm not sure whether they had a preexisting relationship or maybe teamed up against Sothis. Now even in their diminished states the two of them retain an instinctual emnity.
I'm really looking forward to seeing where this goes (but it might take a while since kind of a slow gamer
)!
Sidenote I liked seeing Byleth and Jeralt's interactions after the battle as well. He's concerned for her but less alarmed than I would have expected at her hair and eyes suddenly changing to green. Maybe offscreen he talked to Rhea and found out more about what went down in the past?
Also it's kind funny that Byleth's plan to relax after the previous stressful events basically is to immediately go back to the battlefield and kill people. Kinda exemplifies the whole Ashen Demon thing.