It would be as deadly as today's sniper rifle - i.e. one good hit = death. The only difference is that rifle works at long range and is mostly useless in melee and your weapon would be the opposite.

And just like sniper rifle it would be absolutely useless in warfare, rather than assassination. Even more useless than a rifle, actually, considering difference mentioned. When your position is stormed by 100 soldiers, you'll hardly have chance and time to approach and slash each of them with your vorpalfreeze sword, while rifle would allow you to have at least some kills while opposing force is closing up.