Knightfall
World of Kulan DM
Yep. Being tempted will be a big problem for heroes with Darkness powers. A villain likely won't care unless he/she is the type of bad guy that like to remain in control of his/her abilities and not the other way around.I like your ideas for Darkness. It sounds like guys who want to abuse their Darkness powers will have lots of free uses of extra effort (check out the pink section of the book) to increase their power ranks by 2. That's all kinds of tempting. }
Mutants will have to worry about this too if they have been touched by Darkness, but it won't be as big a problem for them. A mutant that reaches into themselves to tap the Darkness can enhance their mutant abilities for short periods but it cost them part of their sanity/soul.
Mutants and superheroes touched by the Energy don't have to worry about this unless they've also been touched by the Darkness. (A very rare thing.) Such a hero that falls from grace would become the wickedest villain. Aliens would be the most likely characters to be touched by both the Energy and the Darkness (i.e. fraal or sesheyans).
And thanks for the PL guideline. That definitely helps.