>>4867378
In the Japanese version, it's just the Heart of the Land, that thing you need to have to have Metafalica work. When Nenesha was killed, she left behind a Heart of the Land - the one in the Moon Culvert - probably because, like Cloche or Luca, she was in the middle of singing Metafalica at the time. I think that's what Infel used to "revive" Nenesha. Probably what happened is the revival was imperfect and ended up overpowering Infel, or something. I rather got the impression that Jakuri meant Infel was the one being influenced. The conversation you're referring to is something like (sorry for the crappy off-the-cuff translation) ...
まずいね…あの大地の心臓の「魅惑力」にはまってしまったようね。
Jakuri: Oh no ... it looks like she's totally taken in by the power of the Heart of the Land ...
あ…あの大陸創生の時の…!
Croix: Oh ... ! Like during the creation of the continent ... !
そうよ。自らの信念が全てになってしまう、あの不完全の意志の力…!
Jakuri: That's right. The power of that imperfect spirit, whose conviction has become everything ... !
I don't remember what they called it on the demon continent, and I don't think I have a save conveniently placed to go find out at the moment, but 大地の心臓 (lit. heart of the land) is what they say at the end of the game, and "Gaea" is always referred to as 魔大陸 (lit. demon continent) once people know what's going on.
Maybe the Settei Book has more. I haven't looked through the translations much, I'm kind of waiting to just get my own copy.