Soul plasma

Players can harness the plasma from soul fire by using a nearby cauldron and a lightning rod on it to channel it (it's going to have to be struck for the plasma to go in).

The resulting plasma can be picked up with the bucket, but the player mustn't let it flow, because then it escapes into the environment.

Soul plasma source blocks can be used instead of lava as a damage medium, because electrically conductive items sink into it without burning (probably not the most realistic feature, but it would make for a cool gameplay mechanic).