False Knight

Attacks
The False Knight will slam the ground with it's mace, either being a normal slam attack, or a slam attack followed by a shockwave. The shockwave version takes a bit longer to charge.

The False Knight can jump around, attempting to make the Hollow Knight into a Hollow Pancake.

Rocks with orange blobs inside of them can occasionally fall down, they can be hit at the False Knight.

The False Knight can jump then hit the ground with it's mace.

After enough damage is dealt to the armor, the False Knight reveals it's true form: a maggot.

When the maggot takes enough damage, the False Knight will slam both sides of it rapidly in anger. Then it will resume it's normal attacks. The rocks previously mentioned will fall more often here.

Tactics
Your best bet at fighting the False Knight is to keep moving, attacking the armor when you can. When the maggot shows up, show no mercy! When the shockwaves appear, your best bet is to jump over it. It's recommended to get the long nail charm before fighting this boss, as the extra range is needed if you want to take less damage.

During the rematch, the Failed Champion will spam it's attacks CONSTANTLY. And the rocks will fall more often than not.

Thankfully, after taking three phases worth of damage, the False Knight will fall to the level below it and the player. And the maggot will always be revealed.

Trivia

 * The armor the maggot stole actually belongs to the legendary Knight Hegemol (mentioned by Nailmaster Sheo)
 * The reason the maggot took the armor was to protect it's fellow maggots, not because of the infection plauging Hallownest. Unfortunately, the player needs to kill them for the journal entry.

Dream Nail

 * "I'm braver than all of you!"
 * "This great shell...It belongs to me!"
 * On his weapon: "Free at last!"