What happens here is going to depend on how you enter Lost Izalith. If you go straight into Lost Izaltih through the regular route which is right after speaking with Solaire after the Centipede Demon fight, then Solaire will go Hollow and you’ll fight him near the area where the door to the Chaos Bed Boss fight is located. However, if you instead take the shortcut from Demon Ruins into Lost Izalith then you’ll encounter nine Chaos Bugs in the corridor on your way to Lost Izalith. You must kill all nine of them. Do not leave a single one alive, and do not cross into Lost Izalith so that that area loads before you have killed all nine of them. If even one is alive and you enter Lost Izalith, then Solaire will be attacked by one and turned Hollow - so kill them all. If you miss one, then when you come back to the shortcut corridor, you’ll find Solaire with one of the bugs attached to his head, and you must kill him. If you kill all nine of them, then return Lost Izalith before returning to the start of the shortcut area you’ll find Solaire sitting on the ground just outside the door that you opened with the Daughter of Chaos’s Covenant. He’ll be depressed, but by talking to him here, you will trigger his final Summon Sign at the end of the game just before the final Boss fight. To find it, as soon as you get off the narrow bridge and arrive at the final spiral staircase, look left to see his Summon Sign.