But what grabbed me was this special power from the valuable gemstones contained in a Wrath:
1 in 6 such gemstones are permanently imbued with stolen life essence. These stones heal the bearer 1 hit point per day on top of other factors, such as rest and magical healing.
Okay, so you kill this life-sucking semi-undead being that wounds people badly enough to force them to recover by rest or just simple piles of healing spells. You take a gemstone out of it which is "permanently imbued with stolen life essence." And this heals you 1 HP per day.
From someone else's life essence.
Does this add up over time, day in, day out, as injuries are healed by it, slowly letting that personality accrete into yours.
Perhaps, in GURPS, this would mean you eventually add on disadvantages from the new personality. Or eventually add Split Personality.
If the stolen spirit is evil, this could be warping. If it was good - say, of a saintly or genuinely good person - it could be reinforcing your positive nature. Maybe these eventually come with positive traits, too, tied to the bad ones. Or at least, traits useful to the bad ones.
Maybe it doesn't come with a downside . . . but you can use that gem to Resurrect the slain victim, or perhaps just to speak with him, her, or it by Summon Spirit. Or perhaps even without. Maybe it's a conduit to the afterlife in both directions - you heal more easily, but your hold on life is just a little more tenuous because you've got a two-way street to the afterlife. A penalty on recovery rolls when Mortally Wounded, perhaps?
Or maybe it's just me? Have I been given too many poison cookies and frogurts with sodium benzoate in them to just take a gem like that as a reward for killing a monsters?