Out of all the bosses in Castlevania: Belmont's Curse, only the Brittle King is truly optional. There is a tasty reward at the end of it in the form of Excalibur, the greatsword of Arthurian Legend and one of the most powerful weapons in the game, but that and the Brittle King's Arcana powers are the only tangible rewards for this side endeavor.
And he doesn't exactly make it easy to fight him in the first place. His chambers are in Notre Dame, close to one of the Sage's Sigils, but as you enter the boss room, you'll discover Excalibur wedged in stone with a plaque hinting at what you need to do to rip it out. Luckily, by this point in the metroidvania game, you should have everything you need to pass his test.
How to start The Brittle King boss fight
To start the Brittle King boss fight and pass his test, you only need to equip the Builder Arcana, gotten by defeating Talos in the Clock Tower. Once you do, simply use the sword, and the stained glass window behind you will speak. Eventually, it'll get angry, mostly because nobody believes that he is King Arthur from the legends, and he'll crash through.
Since you're here, you might as well know a few things that might help you with this boss fight. Attacking him from the front is tricky unless he's attacking, as he wields a large shield. The good news is that there are lots of points to latch your Arcane Whip and swing across the screen, letting you get a few hits on his back. We recommend you equip the Vile Trinket Relic for this exact reason. If you really want to cheese this fight, the Alucard Sword and Shield weapon, which is in the Main Hall, will also negate his defense because it teleports you behind him with every attack.
As for his attacks, he has a few well-telegraphed sword thrusts that are easy to parry or avoid. Bashing his shield generates glass shards that shoot at you, but they're reasonably slow-moving and easy to dodge. There's also a jumping attack that has a large attack radius, so keep away and jump to avoid damage.
The main attack you should watch out for starts when he holds his shield aloft. The sun shines through, emitting beams of light that hurt you and are tricky to avoid unless you've got quick reflexes.
Around halfway through his health bar, Arthur's attacks will intensify. His laser patterns last longer, meaning a smaller amount of space to hide in, so keep note of where the light will strike next to prevent getting hit too much. At around a quarter health, he will shoot them out from his sides, quickly rising up toward his head. It'll be difficult to parry this attack, let alone avoid it, so if you have the damage output and plenty of remaining health, taking a hit here can set you up in the perfect spot to take him down.