[Flere821]

Originally Posted by OmniDragon

Ain't it also the reason why Yuji merged with SoF in the first place? I think I remember something about Yuji saying that he hasn't found a way to truly protect Shana and thats when SoF appeared.

Yuji technically didn't say that right in that part, but he did thought about ways to save Shana but not managing to find one in earlier parts. Yuji had even told Alastor once if what it takes to protect Shana is stopping the war between Flame Hazes and the denizens then he'll find a way to do it, even if Alastor treated it as a joke at the time. Alastor reacted in horror as Yuji's promise to save Shana ended up as foreshadowing of him allying with SoF and as their enemy in Vol16 Ch3.

Here's the Chinese version from Vol14 Ch3, where Yuji is taken by the SoF:

Spoiler:

这个感触是，金属的——钥匙。
就在他发现这一点的一瞬间——
（——……选 中了么，——坂井悠二——）
响起了一个似乎从遥远的地方，从无底的深处传来的声音。
“！？”
悠二吓了一跳，连忙东张西望地寻找声音的根源。
建筑物的屋顶上空无一人。
于是他把视线移向前放。
“什么、怎么回事——！？”
只见眼前站着一个“漆黑的自己”。
声音继续在回响，仿佛从一个深广的空洞中传过来一般，带着回音——
（你一直在希望自己能够主宰这场战斗……因为是你，才会有这样的愿望……）
（你、你在说什么……啊……）
悠二想把手从钥匙上放开，可是身体却不听从使唤。
在身心都变得冰冷的悠二面前，另一个一模一样的轮廓，象幽灵一般地摇动着。
那影子发出的声音如此的虚空，仿佛象呼啸的风声一般飘渺。
（你……只有你……最为适合……）
悠二从手中的钥匙上感觉到了某种自在式将要展开的预兆，开始恐惧起来。
那“漆黑的自己”变化成平面的存在，慢慢地靠近。那虚渺的声音仍然在继续，就象在确认什么似的 语气。
（和我共同踏上征途的……唯一的……人类。）
悠二被那超出了恐惧的诱惑吓得不禁发出了拒绝的悲鸣。
“漆黑的自己”，象水中的倒影一般的自己再度靠近。
虚无的声音突然混入了感情的火种。一点点的愤怒，似乎在迅速蔓延。
（为何要拒绝？你真正渴望的东西……只有我能够给你……）
声音在颤抖，说的话在颤抖，心也在颤抖——恐惧让身体产生了麻痹。
那“漆黑的自己”，影子的深处，如此的遥远，如此的深沉。
声音强烈地传达到耳内——
“来吧……踏出你的第一步吧……”
（到、哪、里……？）
面前能够看见的只有屋顶的边缘。悠二开始主动地迈开脚步，走向那浮在半空中的“漆黑的自己”，以及倒映出那 个身影的，深远的水面。
一个声音发出了，不是在心里，而是在口上——悠二自己的口上。
“御命王道……”
（御命、王道？）
钥匙已经展开了自在法……可是，自己已经感觉不到恐惧了……
他直往前走，走向那黑色影子的深处，走向那深不可测的水底。
一个声音在脑内如同饥渴一般威胁一般摇撼着——
“没错。是你……希望我出现的——”
（我……所以你、才会存在……）
然后，他往虚空中踏出的脚步，却没有降落到地上。
身影一下子消失了。

SoF showed up and offered Yuji what desired the most, saying Yuji is the only human worthy to be his partner.

Yuji was taken without given a chance to refuse, but in Vol16 Ch3, Yuji stated he accepted SoF's offer to an alliance, so he can save Shana from the fate of all other Flame Hazes who live out their lives as nothing but tools of the Kings of Fuzes as they fight the denizens, and dying in the process with ultimately nothing changing as the world continues to be devoured.

SoF's convinced Yuji his revival as the Creator God and the creation of a new world are the best way to save Shana, and Yuji agreed.