Her boosted power should be beyond what Arthas had as the LK. Original LK and sylvy with her new powers would be fun fight to watch, i'm sure she'd win..but the fight would be a sight to behold.
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It is odd to introduce the new LK and already have him kneel easily to sylvanas. Blizz has some odd writing these days. I'm sure bolvar spent some time being confused and trying to understand his new power and abilities, but we don't know where/how and who he tested them on. This would have been an interesting question at blizzcon. *Where was bolvar testing his new powers?* of course they would most likely give some crappy answer..but would be interesting to hear their reply.