Bouldergeist is a boss in the game Super Mario Galaxy. Its name is a portmanteau of boulder and poltergeist. It is the boss of the Ghostly Galaxy and is responsible for kidnapping Luigi. Bouldergeist returns in Super Mario Galaxy 2 as the fourth out of five Super Mario Galaxy bosses encountered in the Boss Blitz Galaxy, using the same tactics as in the last game. Boldergeist shares its music with Kingfin.
Bouldergeist is essentially a huge column of rocks roughly shaped into a face, and its insides are made up of shadows. However, its true form is revealed when its rock armor has been destroyed, revealing a face with a red uvula hanging from the bottom. In its rock form, it has two enormous stone hands, which are used only in the second half of the battle. Bouldergeist's attacks include tossing rocks at Mario, summoning lines of stalagmites from the floor, and in the second half of the battle, slamming its hands on the ground and punching at Mario. It is fought in a plain circular arena.
This battle practices Mario's use of the Bomb Boo. To get Bomb Boos, Mario must dodge the black rocks that the Bouldergeist throws at him. Once they hit the ground, they will turn into Bomb Boos. Each hit will gradually cause Bouldergeist to lose a bit of its stone layer. Three hits will completely destroy the entire stone layer surrounding Bouldergeist's real form. Once this happens, Mario has to, yet again, spin a Bomb Boo at the shadow's red bubble to inflict more damage on Bouldergeist. However, the fight is not over, as Bouldergeist regains its old stone layer, and reveals its large rock hands. Bouldergeist then has a greater variety of attacks, as well as a larger form of protection. Mario can, however, destroy the hands the same way, but they regenerate after a short time, so Mario has to damage Bouldergeist before the hands reappear again. Mario must once again swing three Bomb Boos into it, and then swing one final Bomb Boo into its shadow form to defeat the Bouldergeist and claim its Power Star. Also, Bouldergeist may shoot golden rocks which reveal a coin. However, it only shoots up to a maximum of 20 coins.
Bouldergeist's attacks consist of a boulder throwing attack that Mario can only run from, and summoning a row of stone spikes out of the ground, indicated by dark shadows appearing where the spikes will be. The spikes will not harm Mario after they are out of the ground and can be broken with a spin attack. After the boss forms its hands, it has new attacks in addition to to the old ones. If Bouldergeist draws back its fist, it is preparing to punch Mario. Bouldergeist cannot change directions once it starts the attack so it is easily avoided and used to destroy any stone spikes. If Bouldergeist holds its hands over the surface of the arena, it will use a slap attack that will crush Mario if not dodged, though it does not cause instant death; this is the only instance in the game in which Mario can get crushed without instantly dying, and after being crushed, it will just vibrate, pop back up after a few seconds, and lose one health point. Also, Bouldergeist's stone spike attack forms two rows of spikes that Mario has to avoid. Bouldergeist's inner form is too weak to attack Mario. It may only flee and summon Bomb Boos.
Later, Mario must defeat Bouldergeist with one wedge of health under the Daredevil Comet to claim its second star. Bouldergeist will no longer throw golden rocks in this battle.
Bouldergeist has unused animations for popping into and out of the ground, and when it does this its true form appears with a somewhat early design.
Names in other languages