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値l fight him near the area where the door to the Chaos Bed Boss fight is located.
Enter Lost Izalith through the front area.
Solaire becomes a Hollow and attacks you near the door to the Bed of Chaos Boss Fight.
Enter Lost Izalith via the shortcut, but do not kill all nine Chaos Bugs.
Solaire becomes a Hollow and can be found in the shortcut area.
Enter Lost Izalith via the shortcut, and kill all nine Chaos Bugs.
Solaire shows up as normal just outside of the shortcut door and you can talk to him to trigger his final Summon Sign.
However, if you instead take the shortcut from Demon Ruins into Lost Izalith then you値l 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 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値l 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値l find Solaire sitting on the ground just outside the door that you opened with the Daughter of Chaos痴 Covenant. He値l 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.
You値l find the shortcut to Lost Izalith in Demon Ruins just outside the room where you fought the Boss Firesage Demon. As you face that room, look to the right, and you値l see a hole in the wall. Use the tree roots to reach the ground floor and then go to the end of the long hall where you値l find a locked door. The way to unlock that door is to join the Covenant of the Daughter of Chaos, and then level that Covenant to +2 status. That will require 30 Humanity to raise the covenant that high. After you raise the Covenant to +2, then you can touch the