Blue Exorcist (2011)

S1E2 Gehenna Gate

Fujimoto and his friends do everything they can to shield Rin from the pursuing demons, but ultimately, Rin becomes possessed by Satan, the most formidable of them all and his own father. Witnessing his guardian sacrifice himself to save him from Satan, Rin resolves to push back by awakening his demonic abilities, fully aware that this choice means relinquishing his humanity.