Ivalice Raids. I still don't get it

[Frizze]

Yup, and this is an important distinction. It shows that the Auracite can be used with NOBLE intentions, and the results are monstrous, but still good-natured beings, unlike the twisted, selfish, vengeful creatures we'd fought up to that point. The battles against Mustadio, Agrias, and Cidolfus are there to test any who would try to defeat Ultima. Anyone incapable of defeating them would stand no chance against Ultima herself. The flashback cutscenes following Orbonne Monastery show the three of them volunteering to take on that duty, in fact.

It's notable that while Auracite was created by Ultima, the stuff isn't inherently evil. It can be used for good or evil, depending on the nature and intent of the user. Ultima herself, for that matter, is presented more as something incredibly alien, rather than explicitly evil. When first encountered, she attempts to resolve the situation peaceably, with an offering of more Auracite - and seems surprised when we don't gratefully accept. It's not hard to imagine that back in the days when she was worshiped, this was a standard exchange for her.

So i was going to add this point in my reply to Silver, but didnt want to muddy the narrative with "side details". In the original FF Tactics game they actually show an instance of the zodiac stones being used for a pure purpose. A side character is killed following a battle, and one of the stones(was it virgo? i havent replayed that game in a while) reacts to her brothers feelings and revives her. No possession, no lucavi summoning - just an honest "miracle". So the idea that Cid and company could have used the auracite to become guardians of Ultima's tomb isnt really that far-fetched - using game logic anyway.