okay so basically, when lumi created the sphera that she traps xia in, (the one with the flow in it) it was unstable and to stabilize lumi had to manually hold it together by giving something (easily interpretted as her vita but its something else)

also the reason lumi was able to make it so xia was functionally the overseer was by arranging it spatially how she understood her own situation to be, arranging the flow as a disc surrounding a center with xia’s vita in the middle

this explains why xia is functionally the overseer, lumi gave her the power with her thing she gave up, and xia can see and interact with the flow directly because this sphera is not the normal kind with spherical shell.

as for how the primi can beat her, it of course involves modotsu

basically after xia betrays invera, the invera primi share what they know and one of those things is that the reason their and the primi have powers is because their vita interacts with surrounding vita inside the sphera, but outside of there, in the palace, they can’t use their powers unless they have vita they can interact with to control. (conveniently, locke’s power)

during their fights, locke speculates on how she could find a way to reveal xia’s vita, and thats why they keep fighting her, trying to basically find a weakness. finally she thinks she found it, the flow that surrounds the palace, perhaps xia is tied to that. however when locke tries to travel to the flow, she find she never gets closer.

using the fundamentals she learned from invera, she knows that the only thing that goes to and from the flow is dissolved vita, and locke resolves to get herself dissolved to allow modotsu to extend an attack through her vita and destroy the flow, hoping that will get rid of xia. (they wouldn’t know this would end all reality, they convince themselves that protecting the few remaining realities they have would be worth it to avoid xia fully realizing her plan)

this works. by getting dissolved and making a sword for modotsu to act as a sort of reference point, modotsu incants that everything making contact with locke’s vita be destroyed. in short fight with xia she stalls until the last of locke’s vita can’t be sensed, then she stabs xia quickly, lets go and completes her attack destroying the flow and xia. however in this process this ends the reality.

i think this is an interesting way to go about things because it keeps things in line with the previously established rules, and stuff.

for why modotsu couldn’t do this before: you can’t reach the flow as is so she can’t attack it directly as a target. she can however dissolve things and she can target vita, and she can certainly make a recursive attack that would otherwise be risky to do in a sphera because she doesn’t know what the spatial relationship between all vita is normally. yes this action at minimum dissolves locke in all remaining sphera.