Well...this is shitty.
MC literal prank war reward got used by the enemy, and for his own decision to watch people and friends are suffering.
This is worse than when Batman had to recapture everyone in Arkham when he could have called Superman for help and finished it in 5 minutes, because in this case Superman is watching the show.
MC made that grail to help step-sis, could have made it dissapear when it was gonna get stolen, or seal the demon god before it tried another trick, is bad writing when the protagonist have more than enough time, power and chance to avoid a incident and doesn't, super forced plot on the reader.