Watcher Knight

Behavior and Tactics
The Watcher Knights have three attacks:
 * Double Slash: Watcher Knight will swing their giant nail twice. Watcher Knight will swing horizontally first and follow it up with an overhead slash. Watcher Knight does move forward slightly with each swing.
 * Roll: A Watcher Knight curls up into a ball of death and rolls along the floor of the arena until it hits the opposite wall. They are immune to Nail attacks while rolling, but can still be damaged using Spells.
 * Bouncing Roll: Watcher Knight curls up into a ball of death and bounces twice. When they do this attack, Watcher Knight will attempt to target The Knight, which can mean that Watcher Knight may simply bounce in place rather than travel around the arena. The first bounce takes Watcher Knight to the top of the arena and the second bounce takes them only to the middle. Similar to the Roll attack, they are immune to Nail attacks while bouncing, but can be damaged using Spells.

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 Cocoon 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

 * The Watchers Knight were the sixth boss to be revealed through the kickstarter under the name of "Black Knight". Through this and other  promotional material it can be inferred that there was originally only planned to be one knight.
 * 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 Dream Bosses, The Radiance, and Nightmare King Grimm.
 * They are referred to as "Black Knight" in the game files.
 * After the Lifeblood update the bodies of the inanimate Watcher Knights have been rearranged.

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