Ideas for a Soul Bond blade?

I've been talking to my players about some ideas for cool weapons and my gunslinger brought up the hostage mechanic in .... A first person shooter, where you could take a hostage and all that. Now I know mechanically this could be be just a grapple check. But he seemed excited about the idea of a "soul bond" karambit that was maybe a way to share hp.

Any thoughts on mechanics to make this fun cool and balanced