From D&D Wiki
A few things:
- It's unclear what the "midweight" property means.
- Realistically, an odachi or nodachi would make must sense just using greatsword statistics, as strikes are almost made with two hands, and could hardly be used with finesse in one hand. See Weapon Alternatives (5e Other).
- Considering the special property ("you lose proficiency for all other martial weapons"), this might make more sense as a (cursed) magic item. Without the property, this weapon would needless to say be way too strong as a normal weapon—even with it, I feel like it might be.