The Pantheon of the Knight is the 4th and final of the curated Pantheons in Godhome. It is only unlocked once you beat the previous 3 Pantheons. It is located on a sealed platform to the east of the other 3 entrances, and will play a short unsealing animation when you get close enough to it.
This Pantheon is a similar challenge to the Pantheon of the Master, but the final boss is a step above, and one of the hardest fights to master in the game alongside Nightmare King Grimm and Absolute Radiance. This Pantheon also includes all of the bosses in the game that deal 2 Masks of damage per hit besides the Nightmare King and Radiance, and the Dream versions of other bosses from the Hidden Dreams update, namely Lost Kin, White Defender and Failed Champion.
Pure Vessel is the final boss of the Pantheon and the Knight that it is named after. It is a version of the Hollow Knight boss without the infection, retaining it's other arm and gaining abilities to summon swords, generate explosive areas in phase 2, and lash out with Void Tendrils in the 3rd phase. It's faster, hits harder and stays stunned shorter between phases. Once you battle it once, it is unlocked in the challenge room in Godhome which can allow you to practice it to learn it's attacks and telegraphs and improve without needing to fight through the Pantheon each time.
The best strategy with Pure Vessel is to just focus on dodging and not being greedy, going for single hits during attacks and not overusing your dash so you always have a Shade Cloak dash ready to get safely through the sweeping arc of swords and get a few hits in while the rest of the arc is completed.
A Charm build of Fragile/Unbreakable Strength, Quick Slash, Mark of Pride and Soul Catcher tends to be my go to, but you could swap Mark of Pride for Longnail and use Shaman Stone instead of Soul Catcher if you want some more oomph behind your Spells. I would avoid overchaming here since most of the bosses would be hitting you for 4 damage, unless you are very confident with your placement and dodging.