Watcher Knight

Behavior
The Watcher Knights have three attacks:
 * Watchful Slash: The Knight swings its massive Nail twice in succession.
 * Rolling Crusher: Curls up into a ball and spin-charges the player.
 * Bouncing Crusher: Curls up into a ball and jumps several times while spinning.

Tactics
Since the Watcher Knights attack in groups (there are 6 of them in total), it's best to not get cornered by them. It's best to jump above them and hit them from the air, as their spin jump is their only counter to aerial attacks.

Since they have fast attacks, Quick Focus is highly recommended for this fight. Another tactic is using Lifeblood Charms for a health boost, and Stalwart Shell to increase invincibility time, then damage-race the Knights down while face-tanking their attacks.

Directly left of the arena there is a breakable ceiling (accessible via Mantis Claw) which reveals a secret area containing the anchoring of a chandelier. Striking it with the Nail will drop the chandelier, crushing one of the corpses below and permanently reducing the amount of reanimated Knights from 6 to 5.

Trivia

 * Given their name, they are presumably the deceased elite bodyguards of Lurien the Watcher.
 * Combined, the Watcher Knights have the highest total health of any other Boss in the game except for The Radiance.