Yes, I know, that's why I said that they catch him when they kill the Phantom
If it was separate entity, it would be visible and/or drop Phantom Membranes elsewhere.
It's not, so either Phantom is:
1. collective of:
a ) a completely invisible membrane carrier;
b ) a hallucinated image of aggressive attacker that gives heart attack, somehow more often unarmored players and less so to ironclads, shielders, potion drunks and golden apple eaters and makes the survivors somehow more vulnerable to regular damage and disappears together with the carrier;
2. regular flesh-and-blood undead beast that damages the player character with physical claws and teeth and is warped in valuable membrane.
At that point two of these options are indistinguishable... but my common sense is telling me that the option 1. is really wacky logic bending and way less probable than option 2.