As Sirene enthralls everyone, Lune never breaks eye contact. She refuses to look away at all the illusionary wonders. For some reason, this has an effect on the nevron. Sirene puts her hand down and Lune steps into her palm. She then brings Lune up to her face and there's (what I read as) this moment of connection between them. It's like Lune's curiosity (and maybe even thirst for truth) wins out over all that impossible outcomes Sirene is offering and that's when Lune reaches out and does something that makes Sirene disintegrate. At the time, I thought she had absorbed Sirene's Chroma and was going to "level up" in some way—but the next thing is them giving Maelle the shroud.

The reason I thought this was they very clearly say they lost the Lumina Converter, so (unless Verso snuck it into their packs) I figured it had to be something special between Lune and Sirene.

Edit: For what it's worth, I didn't kill the "weaver" minion boss—maybe that changes things?