>>535412471
Realistically, both Nibelung and Phanes are some sort of Supreme Deities of their respective worlds, matched in their power overall, with Phanes' world likely being destroyed by Abyss in the past. As such, when Nibelung returned with Abyss powers, he effectively presented himself on a silver platter as Phanes had likely developed some anti-abyss measures before, with the Nails being the realization of those measures in Teyvat.
This would actually neatly mirror Xiuhcoatl and his self-afflicted weak point. If not for his Eye, Xbalanque wouldnt be able to beat him, and without bringing Abyss with himself, Nibelung wouldn't be beaten by Phanes who was weakened after the 40 years of warring against Sovereigns.