I send this challenge to those who think they're brave enough to adapt this story. A story about One Punch Man and Fate Crossover.
The story is of course about Saitama getting summoned into Holy Grail War. But not as one of 7 common class. But as a Hero Class. A Hero Class is summoned when the War is dangerous enough that it definitely will expose the existence of magic and even destroy the world at worst. Since we know Saitama is OP you might also add any other elements. But if possible keep it only in the range of OPM and Nasuverse. You might change this as you see fit.