Over on the forums, Anders asked:
A special type of undead are gastar, who consist entirely of psychic energy. This means that their HP = Will, and each time they get damaged they lose Will as well. And when they reach 0 they are destroyed. The second part is easily covered, but what about them losing Will when they lose HP? How do I model that?
At first I thought Modifying Other Characteristics (GURPS Power-Ups 4: Enhancements, p. 10) would have been the way to go. When I was writing my article “Mind and Body” in Pyramid #3/69: Psionics II, I used “Based on Will” to substitute for ST for basic damage for the psi-sword ability (see p. 18 for that ability). So using Modifying Other Characteristics we add “Based on ST, HP Variant, +20%” directly to Will so that you could use your base Will as starting HP. So if you had a Will of 11, your starting HP would also be 11 – even if your ST was 0 (like many insubstantial beings or spirits). One thing I noted in the thread was something that I screwed up on – you wouldn’t lose any levels of Will if you lost your HP anymore than you’d lose levels of ST when you lost HP. Using this method would result in HP costing 1-point per level – it’d be cheaper than regular HP by far. Of course, you’d need to spend some points starting to get your HP to your base score so you wouldn’t see if cheapening the cost till higher levels.
Psychic Projection: You are a spirit, “psychic construct,” or similiar being. You base your HP on your Will score instead of your ST. You may buy additional HP at the usual cost, and losing HP has its normal effects, but when resisting any effect based on your Will, use the lower of current HP or Will to determine your roll. Add Fragile (Unnatural) if you dissipate once you are reduced to -HP. If you based your HP on your Will, but have no other effects for lost HP, then this is a -0-point feature. Otherwise, this is worth -20 points.