Verethragna ~Seisen no Duelist~

A card game called Verethragna has taken over the world, and losing at Kokuryou Academy carries real consequences. Tatsumi lost a hidden duel to the chairwoman Methuselah and now can't leave the school; his son Akira enrolls to climb the duelist ladder and settle the debt in his place.

The rules stop feeling fair once Akira wanders into the woods and gets ambushed by monsters no card game prepared him for. A summoned card, the dark goddess Kisskill, saves him and stays at his side as he fights his way up through the ranks of the academy's 21 Knights toward the title of Duel Emperor, with Methuselah's real agenda and the truth about Verethragna's other worlds hanging over every match.

Play is built around turn based duels against monsters and rival students, with the card game framing standing in for combat and management layered underneath as Akira builds up his standing at the academy. Losses aren't just a scoreboard problem: the school setting comes with its share of humiliation, cheating rivals, and monster girls who don't fight fair, so climbing the ladder means both winning matches and surviving what happens when you don't.