AstralLein
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Here's what I think: Biara's devilish chain would suit Solmyr better because this chain deals air damage (doesn't affect units with air immunity) and is enhanced by the Trident of Titans, and so on. That is, this ability is not related to fire and demons, but to air, and Solmyr's spells are related to lightning chains.
By the way, I completely agree! Considering that he is, after all, a heroic primordial, he needs a more powerful ability. And Biara is just a succubus. She could have some kind of demon summoning at the end of the week (she likes that:D).