Here's some weapons that I used in one of my Bloodborne themed campaigns a few months ago. Their effect are fairly simple and I tried my best to emulate the in game effect of transforming the weapon.
They are only uncommon, because they are regular old weapons, with no bonuses. But they allow to do cool moves as a bonus action on each of your turns after your action, so they couldn't be common either.
As you may or may not know, I'm working on my Soulsborne and Lovecraft inspired D&D setting, you can check it out here!
If you join the list you'll get access to the Oath of the Eldritch Hunt Paladin. And on top of that you'll get a free moon themed d20 if you back the project when it launches! (almost 10k on the waiting list already, which is crazy to me!)