Mantis Lords

Battle
It is recommended to upgrade your nail at least once or twice before facing the Mantis Lords, otherwise this battle can be very time-consuming.

The Mantis Lords will not initiate battle until the player interacts with them to challenge them. At this point, a steel cage will prevent escape from the arena, with spiked pits to the left and right of the cage. If the Knight stands too closely to the spiked pits, the Mantis Lords' attacks could push him into the spikes.

The battle begins with a single Mantis Lord against the Hollow Knight. The Mantis Lord will execute one of three attacks, disappearing in a blur of speed moments after each attack. The player must react quickly to dodge the attack, then strike the Mantis Lord before it vanishes. As the Mantis Lord loses health, the attacks come faster and faster. The best moment to heal is during the spinning blade attack. As is the case with most bosses, the Quick Focus charm is immensely helpful, despite its high cost.
 * The Mantis Lord appears on one side of the arena and dashes to the other. Should be jumped over.
 * The Mantis Lord appears above the Hollow Knight and attempts to impale him from above. Should be sidestepped.
 * The Mantis Lord perches on the side of the steel cage, throwing a spinning blade in a boomerang pattern. Depending on whether the Lord is close to the ground or higher up, the blade's trajectory changes (the former demands an immediate jump, the latter will pass above the Knight's head at first, but can hit him as it returns).

Once the Mantis Lord has been defeated, it will return to its throne. At this point, the two remaining Mantis Lords will engage in tandem. The battle remains fundamentally the same, but the attacks come far more quickly and the patterns of the spinning blades change (notably, one of them has a safe spot in the center of the Arena, making it a very good opportunity to heal).

Once one of the two Lords goes down, the remainder of the battle becomes significantly easier, but since there is no visible difference between the two, it's impossible to focus on one over the other.

Trivia

 * Quirrel suggests before the fight for you to upgrade your nail. This could suggest he knows how powerful the Mantis Lords are.
 * If you look closely, there is a fourth broken throne in the room you fight them in. The previous inhabitant of the throne is fought in the form of the Traitor Lord.