Help making Powers and Threshold Magic work together

[Varyon]

Originally Posted by dataweaver

Also, look at GURPS Horror. There's a section in there about “Power Corrupts” (pp. 146–148) that specifically addresses this, talking about how various systems of “delayed cost” exist in GURPS: Black Magic/Spiritual Assistance, Meditative Magic's “point debt” in GURPS Fantasy… and Threshold-Limited Magic; and it suggests “Corruption” and a general term for what these “benefits now; costs later” systems track. While it's written in the context of “making supernatural powers inherently evil”, that's something that can be reskinned.

It then goes on to talk about how these systems can be repurposed to apply to supernatural powers other than magic, such as Enthrallment or Imbuement skills, Effect-Shaping Ritual Path Magic, and Powers. In particular, for Powers it suggests applying the Corrupting Limitation, which in simplest terms causes the ability to generate a point of Corruption with each use for every point that the Ability's point cost is reduced: technically it's a –20% Limitation; but in practice, start by deciding how many points it reduces the Ability's cost by and reverse-engineer the percent discount from that. Relabel that from “Corruption” to “Tally”, and you're set.

And there's also the suggestion of converting the energy costs of things like Temporary Enhancements and Trading Fatigue for Effect into Tally buildup. I'd also add “Using Abilities at Default” to that list.

I'd need to look back through Horror (and Thaumatology for that matter), but I'm pretty certain Corruption is a lot worse than Threshold. IIRC, Corruption doesn't really go away on its own, you need to basically perform good deeds or receive absolution from a higher power or something similar to erase it. Also, instead of getting too much of it risking a (minor at first) Calamity, getting too much automatically results in you basically losing character points in the form of gaining appropriate Disadvantages. I think using Costs FP's -5% for each +1 to Tally is going to be more appropriate than Corruption -20% (in fact, I think -5% for +1 Tally is probably more appropriate than -5% for Costs FP really is).