Yes, I think Agares has a chance of defeating the One. Was Mulcarn not destroyed by a being that he had the power to create? Couldn't it be the same for the One.
You're making an important mistake here. Kylorin didn't kill Mulcarn. The Godslayer killed Mulcarn. Kylorin was merely the instrument that delivered the Godslayer to Mulcarn. And the Godslayer is merely the physical manifestation of an accord between all twenty-one gods, including Mulcarn. You could say that, in a sense, Mulcarn killed himself, as his twenty comrades rained killing blows upon him.
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I'd have to say that I personally wouldn't worship The One. Sure, he's the one true god, but he doesn't seem to mind people worshipping the angels. I'd revere him if I knew about him, but I wouldn't be like the Luonnatar. The trouble is that The One is mostly irrelevant to the average Erebusian. He's the overdeity and the father of creation, true, but he's basically a god of deism: He created the multiverse, then stepped back and watched things go on their own course. Yes, he'll eventually return, but for now he's mostly an interesting historical note (unless you're the Luonnatar--I'd
love to be one of those guys, but I'm assuming we're not important religious figures). The other gods could actually affect me and my life.
I'd have to go with Sirona. Spirit's a pretty good sphere to concern yourself with. Besides, she's pretty nice to her followers, and I know we're not supposed to factor civs into this, but if her most devout disciples are the Elohim, that's a pretty big plus as far as I'm concerned.