The scythe came from his inventory, because suddenly the author remembered this existed. But not before making the MC lose his super high level items -.-'
@Rapala101: He has a magic inventory that he spent 6000 yen on which can store anything. The scythe was in there. Even when it was first introduced he explained it was a miss item from an event, which means it's not all that special but useful when he needed to (rarely) fight melee over magic.
So to everyone else: No, it was not forgotten. He's a magician class. It's like a having a Warrior class fight with a dagger instead of a large axe/sword: Yeah they can use it, but why would they if that's not what the character was designed for?