Xero

Xero is a warrior spirit who can be fought in the Resting Grounds. In life, he was a traitor who rebelled against the King of Hallownest.

Attacks
Xero will initially spawn 2 nails of light next to himself. These nails will home in on the player's location and then fire, but will not track after they are initially set in motion.

Once the nail is fired, it will return to Xero several seconds later, like a boomerang. It can harm the Hollow Knight on the way back.

Once enough damage is dealt to Xero, he will spawn more nails (four in total) and fire two in quick succession with every attack.

Tactics
If the player stands slightly away from Xero when they fire their nail, then dashes to directly below them in the first phase, the nail can be easily avoided.

The player can heal easily by running off the platform and to the far left of the area, where Xero's attacks will not reach. It is even possible to stand directly below the elevated platform and not become the target of Xero's attacks.

Trivia

 * Xero's ragged cloak is somewhat similar to the Hollow Knight's cloak, but also bears some similarity to The Radiance's wings.
 * Xero comments that he thought strength would grant him freedom, and mentions dreams of cutting the plague with his nails. This suggests that his rebellion against the King may have been orchestrated by the Radiance, who manipulates dreams and grants one great strength in return for losing one's will.
 * His red armor, use of energy swords and name suggest that he could possibly be a reference to Zero from the Mega Man X series.