Princess Cannon

The Princess Cannon is a special weapon in Splatoon 2. It is not part of any main weapon's set, and is instead only available in the final Shifty Station layout, MC.Princess Diaries, where it appears in the middle of the stage when there is one minute left until the battle ends. It is similar in appearance and function to the from Splatoon, although the beam is much larger and it takes a little longer to charge up.

Octo Expansion
The Princess Cannon is used in Turf War against Commander Tartar in the Octo Expansion.

After the Turf War, the NILS Statue still fires its laser. Pearl uses the Princess Cannon to counter the laser. As she fires the Princess Cannon off, the score screen shows up, showing the turf war percentages. Pearl wins by 888.8%, and the meter cracks, similarly to how it does in Ranked Battles.

In MC.Princess Diaries
For the last day of Chaos vs. Order, MC.Princess Diaries was an exclusive stage for the Splatfest. When the clock hit one minute left, Pearl arrived in the center of the stage and waited for a player to get close to her. When a player got close to Pearl, she gave them her Princess Cannon and allowed them to aim it. Pearl helps the player to fire it and leaves.