You have 4 kinds of staves with Restoration, Fire, Cold, and Lightning; then you have 4 classes that seem to also follow that path except for one... Dragon Knights have a skill tree dedicated to dealing Fire damage, Sorcerers have a skill tree dedicated to dealing Lightning damage, and Templars have a skill tree dedicated to Restoration.

Nightblades however don't have any specific elemental properties, dealing neutral Magic damage. Cold damage seems like it would fit perfectly, the "Cold touch of death" being assassins and having their own chilled effects would make them more effective in combat and give them an additional utility boost. I would say apply it to the Siphoning tree, you are after all taking the warmth of life away from your target, so when you deal damage with those skills deal Cold damage instead of just neutral Magic damage and apply chilled effects.