Big Boo as seen in Luigi's Mansion: Dark Moon

In Luigi's Mansion: Dark Moon, a Big Boo is one of the three bosses fought in Treacherous Mansion, the final mansion in the game. Luigi and E. Gadd were tricked by King Boo to come into the miniature train station inside the Train Exhibit, because E. Gadd thought that King Boo was there along with Mario's painting. Ten Boos are inside a mini train display in which they come together to form into Big Boo. The 10 Boos that assemble together are the only ghosts in the game who are not sucked into the Poltergust 5000, as they are trapped in the trains' cages and later placed in the Boo Canister by E. Gadd.

In battle, the Big Boo has three attacks. Its first attack involves it trying to crush Luigi from above. It attempts to crush Luigi three times in a row. If it misses all three times, it is momentarily dazed. Its other attack involves hiding halfway in the ground and attempting to charge into Luigi from there. If it misses Luigi and hits the moving train, it is stunned. However, if it misses Luigi and the train, it just floats back into the arena. When separated into individual Boos, they can ram into Luigi. The third attack happens only if Luigi pulls back on Big Boo's tongue for too long, in which the boss will recover and ram into Luigi. This attack is also used by the Boos.

To defeat Big Boo, Luigi has two different methods of attacking it. The first one involves avoiding Big Boo while it slams into the ground three times before becoming stunned. Luigi has to pull Big Boo's tongue and launch it into the drill of the moving toy train. The other way is to make Big Boo dash slightly in front of the train. When the Big Boo hits the drill on the train, he splits into ten Boos. Luigi must pull their tongues and send them flying towards the carriages of the train. When one of them hits a carriage, that Boo gets imprisoned inside the carriage and is unable to escape. Once all of Boos have been imprisoned in the carriages, the mission is complete and the Pixelator takes Luigi out of the train display and back to the Bunker. Afterwards, E. Gadd Pixelates the Boos into the Boo Canister.

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