Wait this isn't in the singleplayer? Not available to be bought and worn by the player? It is a customization option in multiplayer for the Vlandian Knight.
From what I've read the Rhompaia was used by the Thracians and was basically a slightly longer Falx (which was used by the Dacians) on a quite long handle. But the blade was still larger than the handle.
Honestly even with incredible micro managing they often if not most of the cases perform worse than normal falxmen. All that micromanaging is also going to drain your focus and make you perform worse in managing the rest of your army.
It's true they can be devastating but I can guarantee you in 90% of the cases the normal Falxmen will be even more devastating