Boss
How to beat the Brittle King
Arthur, not a nickname you can skip
The Brittle King is Arthur. He is in the Notre-Dame Chapel. Listed health is 11,000 and experience is 500. Konami's character page does not include a portrait for him, so this page uses a gameplay still rather than a fan illustration presented as official art.
GameRant's route says you purify the Sonata and Concerto pipes, defeat Death as The Shadow, pick up the Feat of Strength clue, and equip the Fist of Talos. Drawing Excalibur starts the fight.
The reward
He leaves the Stained-Glass Shield and Excalibur. Fextralife lists Excalibur at attack 44, which is their highest weapon figure. IGN's tier numbers do not use that scale. This page calls Excalibur the top of the two-handed line and does not mix the two attack tables.
What I ran into
I tried to open the fight with the sword still in the stone and nothing happened. The Feat of Strength clue was the missing piece, and it is now the first bullet I would have wanted on the day.
Questions
Where is the Brittle King?
The Chapel in Notre-Dame. His name is Arthur.
What do I need equipped?
Fist of Talos, after the Sonata and Concerto pipes are handled and Death has been beaten in the Choir.