Yeah, this bit here. That's referencing her emotional state, from Katatsuki Kouhon: My point is getting rid of whatever caused the Luminary phenomenon is part of the proper function of the Luminary. Hence why it's a 'retribution mechanism', something that's also stated earlier on. But you're right, it might be an Arc only thing, but there are also statements from the Church saying you shouldn't carelessly destroy a TA's body because there is the danger of them turning into a Luminary, and that's a problem: An overly emotional state and hanging around too long are also problems. There is no question the form isn't meant for fighting (especially prolonged periods). I think they are a bit less versatile than Marble Phantasm. Since Fae Domains reflect the 'user's own truth' or however they were described as, as opposed to 'anything you can imagine'. But yeah, they are somewhat equivalent.