Ivala to the Original Lustlord (The one who are believed to be at God level), is yin to a yang. You could say they're siblings. Their power counter each other. Their relationship is not bad, maybe even more than simply sibling

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With two balanced power of both corruption and purification, comes the third wheel. A god who govern Chaos (yeah it's cliche, i'm not as good as Sierra, okay?) who lust not for destruction nor purification, but disorder and confusion. Naturally, this god despise both Lustlord and Ivala, and the latter two understood that they need to stand to Chaos God.
The cliched grand battle enrages, as a being exist solely to destroy the balance between Lustlord and Ivala, The Chaos God's might rivals The Pair's (both Lustlord and Ivala). It clears that the battle conclusion can only be achieved with unorthodox way. And yeah, long story short the Chaos God is disintegrated but not killed, but in expense of disintegrated Lustlord, which Ivala keeps from truly gone by dividing The Lustlord existential essence and encasing it with her last remaining power to form mysterious surge of corruption's energy, which will be referred as Shard in the future.
This is all part of The Pair's plan, to truly erases Chaos God, by bit by bit scraping his power while he's disintegrated, giving it to mortal, and let that mortal dies along with the power fragment eternally separating it from the Chaos God. Yes, this is my speculation about the truth of the chosen, why all of them are assholes. Ivala deceives her follower saying that is her power, while in truth that's not the kind of power she grants (I think it should be more like Carina's) plus she's not recovered to her full power yet.
Then, after the Chaos God completely erased, The Pair's plan enters it second phase, where Ivala will begin resurrecting The Original Lustlord, that is by collecting all (or maybe just a right amount) Shard and let it be possessed by ONE right person, which will turn that person as a vassal for The Lustlord to possess and take over. This is my speculation of why despite the whole world in jeopardy (Thanks to the fact that with Lustlord disintegrated and Ivala not in her full power, the balance could not be maintained), she choose to remain passive. Yeah, it because she still have duty, to completely erases Chaos God's existance essence by creating even more "Chosen" to be eliminated.