It starts after that one battle – the one where they all think the Infernal Multitude is targeting that artist because she’s an old-soul, but it turns out she has one of the Shards as well.
(In fact, Luna’s pretty sure – and Astra agrees – that even the Multitude didn’t realise she had the Shard, making it a case of the world’s worst timing when the wretched thing activates and traps them all in its Broken Reflection just when the Guardians are winning.)
So there they are, all inside the Reflection: three Celestial Guardians, a battered cadre of fiends, and, as it turns out, Kestrel.