Spelled wrong I know, but the faq says I have to talk to a kid running around the cathedral in Daath, but he isn't there for me! :/ I'm at the part where *SPOILERS* I'm bringing the new Ion back to Daath *End Spoilers*
You should already have Nebilim. It's an automatic drop from the Blade Rex in Meggiora Highlands. You get the Lost Celesti by talking to Peony after Tritheim and Pere. After that, and after doing Ion's funeral, returning to the Cathedral in Daath trigger a scene in which Florian runs in front of you carrying the Heart of Chaos. He'll ask you to play hide and seek with him. The reward for finding him is the Heart of Chaos, Anise's Catalyst Weapon.
Dawn_Wanderer: If Endrance was a pizza, what toppings would you put on him? Slayer0: Shiro, then watch them fight over who should be on top of who.