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 its true form: a maggot.

When the maggot takes enough damage, the False Knight will slam the ground with its mace, alternating left and right 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 pops out, get in as much damage as possible. When the shockwaves appear, your best bet is to jump over it. Longnail can be helpful if you're having trouble..

After you obtain the Dream Nail, you can find a hidden passage with the only two maggot in the game and the corpse of the False Knight. If struck with the Dream Nail, you can rematch the False Knight, who has become the Failed Champion. The Failed Champion will spam its attacks near constantly, and rocks will fall quite often.

Thankfully, after taking three phases worth of damage, the Failed Champion will fall to the level below, 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 its fellow maggots, not because of the infection plauging Hallownest.

Dream Nail

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