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?
this author sucks bro, in the beginning he said he can't mention how he knows the dungeon because it wouldn't make sense, but now he just say what the lower levels are and it's okay...