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DarklingThrush: Is this really new? I think we know the Void is a more potent / greater threat since a long time ago. The possibility of a Void Titan (supposedly stronger than anything else in creation) alone makes them that dangerous, not even counting the massive threats from the Void Lords. The Legion is powerful, but Sargeras is allegedly < Void Titan (and there is only one Sargeras) and they would actually be much less of a threat without Sargeras & his commanders' leads.
(Edit: oh, didn't realize that MMO-C automatically tag the OP in now when you put a "@OP" in your post)
They actually don't care *that* much about mortal races, to be fair. They might be benevolent and they do care about their followers, but their ultimate goal is to combat the Legion and the Void. If they are that altruistic and good, they'd have a lot of problem with Illidan. Yet, in Illidan, even *before* knowing he was to be a champion of Light, the Sha'tar (A'dal's group) didn't think they need to wage war with Illidan, as "
At this moment Illidan opposes the Legion. He is its enemy. We take advantage of this to gather our strength". And that's when Illidan was already kidnapping & creating Fel Orcs, enslaving Brokens, etc.
Fel is most destructive in general. I'm not sure if that makes most powerful energy - possibly if you are talking about destructive output only.