Kiuas. Fuckin' badass power metal. Every song is good, but try "Warrior Soul" first. The vocalist uses clean vocals (95% of the time), but he's got one of the best voices I've ever heard.
Pyramaze and Evergrey are also power metal bands that usually deal with darker themes in their music, although they sound more like straight-forward power metal.
Ihsahn - The Adversary (not necessarily Power Metal)
Bal Sagoth (not power metal but has Power Metal style instrumentation)
Kadenzza - The Second Renaissance
For those of you who don't know what this is, it's Satan's Host - Metal From Hell. Fronted by Harry Conklin of Jag Panzer fame. The most evil album that could ever be called Power Metal.