Yuuji already has a power capacity equal to the Kami-level Guze Kings (Alastor, Snake of the Festival and a third unknown). He also has a sword that injuries anyone who comes in contact with it, and a ring that blocks out offensive magic. The last two together make him nearly impossible to hurt.
After he hands Sabrac his ass, the Snake of the Festival takes a liking to him. Snake was sealed in a dimensional prison centuries ago by all the other Guze and Flame Haze, but he was controlling the Knight of the Silver Flame which he was planning to use as a vessel when he roke free.
Instead, he chooses Yuuji. So using the Reiji maigo as a channel, he enters his body. Yuuji and Snake merge becoming The Snake of the Festival, Sakai Yuuji, leader of Bal Masqué, owner of the Reiji Maigo, Seirei Den, Haridan, and a buncha other Hougu, and the Goddamn Batman.