[For some reason, this ended up in Trash after publication. Weird. Reposting…]
Blame this.
Halo-loam
Sometimes, a Saint’s body remains undecayed after death. …Sometimes. It’s actually preferable that it doesn’t, from the point of view of relics engineers. There are a lot of undeniably useful things that you can get out of a genuine Saint’s corpse.
What’s that? “This is a gruesome topic?” No, gruesome is what happens when an Infernal portal stabilizes on this plane of existence for too long and a Fear Squad of Hell comes barreling out. They like to play with their food, and they think that food that doesn’t talk isn’t really food. So recycling sanctified remains isn’t exactly what you’d call ‘horrible.’ In fact, if you could manage to contact the Saint in question, he or she would undoubtedly even guide you through the recycling process. Because, again: Fear Squads. Hell.
Now that we’ve established that… turns out that halos are in fact real. Almost intangible by mundane standards, but real. And, after death: well, the holiness will merge with whatever organic matter that the halo is touching and create Halo-loam. Which is potent stuff. If you get it on your skin… well, regular humans get temporarily ill as all the evil in their souls metaphorically shrieks and tries to get out of range. Supernatural evil creatures tend to spontaneously detonate. Demonic supernatural creatures simply take triple damage and lose whatever supernatural protection against damage they might normally have.
Alas, Halo-loam is rare, hard to process, and gets used up. Still a thoroughly nasty surprise, under the right circumstances. …Assuming that you can call concentrated Essence of Goodness ‘nasty.’