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 Killer Wail from Splatoon, although the beam is much larger and it takes slightly longer to charge up.

Appearance
The Princess Cannon is similar in appearance to the Killer Wail. However, the Princess Cannon has a larger version of Pearl's crown from the Octo Expansion on the top left. Additionally, the parts that would change color to match the user's ink on the Killer Wail are always pink on the Princess Cannon to reflect Pearl's ink, and there is a gold sticker of Off the Hook's Splatfest logo on the sides. There are also some Inkling words written on the inside of the megaphone in Marina's ink color.

Octo Expansion
The Princess Cannon is used against Commander Tartar in the ending cut-scene of the Octo Expansion.

After the player successfully completes the game's final boss battle, Turf War, the NILS Statue still manages to fire its giant ink laser at Inkopolis out of the cannon in its mouth. Pearl uses the Princess Cannon to counter the ink beam. As she fires the Princess Cannon, the gauge from the score screen of multiplayer Turf Wars appears, showing the percentages of ink covered like normal. Pearl manages to score 888.8%, and the meter cracks, similarly to how it does in Ranked Battles.

MC.Princess Diaries
MC.Princess Diaries is the final Shifty Station layout, debuting in the final day of the Chaos vs. Order Splatfest. When there is one minute left of the battle, Pearl slowly descends from Off the Hook's helicopter and lands in in the center of the stage, then waits for a player to get close to her. When a player gets close to Pearl, she gives them her Princess Cannon and allows them to aim it, then helps them to fire it. Using the Princess Cannon is considered using a special weapon for the purposes of the special gauge, so the player is always left with an empty special gauge after firing the Princess Cannon regardless of how much gauge for their own special weapon was filled prior to aiming the Princess Cannon. The Princess Cannon can only be used once per battle, since Pearl leaves for the rest of the battle after it is used.

Trivia

 * Pearl likely has multiple, as seen in the Chaos vs. Order Splatfest Promotional image.