dnd 5e – What is the rarity of my hand-to-hand combat weapon that disappears in critical hits?

Let's be honest, it does not really matter if a weapon is Common, Uncommon or Very Rare, since they are simply terms that the DM can use to determine when a character must obtain an object, and it sounds as if it were an object that ; I really like that your players have. What really matters is how desirable an article is, and this is not desirable at all.

At first glance, it may seem like an incredible weapon for someone, but that will quickly disappear quickly the first time the fighter accidentally vanishes the great evil in another dimension for a minute.

Because you have no control over when it happens, this weapon will take you to a lot of frustration when it happens at the wrong time and no one will be able to attack what they were fighting for a full minute, which will cause concentration fans to exhaust themselves and make Concentration spells that were already affecting the end of the creature because suddenly they are in another plane of existence.

It will launch carefully planned attacks in complete disorder and the effects are completely random to be of any help. You can not even use it to escape quickly if things go wrong, because you'd have to be able to get a 20 on demand somehow, and if you can criticize to your liking, you can also kill whatever you are. We were struggling to crícar sometimes.

Do you want to make it attractive?

If you want players to really use it, giving them the option to activate or deactivate this effect would help a lot to make it more attractive. It will remain extremely random, but at least you will not accidentally ruin your group's plans.

You'll probably also want to turn it into a +1 weapon, at which point it could be a pretty decent rare weapon for a fighter, and the effect could be useful once in a dozen sessions.