Effects of using black separation crystal?

These are the uses of the stone (Informative to those who doesn't know):

• Sends back phantoms in your world (not invaders) to their world.

• You being the phantom, this stone sends you back to your world (Also works if you're an invader).

• Vanishes NPC phantom allies (beware as their summon sign won't appear again even after resting at the bonfire. A friend of mine mentioned that it shows up again if you reload the game.

• If you're the host and use it, you can see your victories as a phantom.

Actual Answer

The reason for the "beware fickle use of this item" warning is because if you use it either as a phantom or as a host to send back people, you are committing a sin. I've only tested it as the host sending someone back and it did add me sin.