Goomboss Battle

Goomboss Battle is a special level in Super Mario 64 DS found in the Rec Room, locked behind an 8-star door. It is a large wooden area with toxic gas. This level's boss is Goomboss, and beating the level unlocks Mario as a playable character.

Geography
This level is a large plane covered in toxic gas with several tree stumps that can be used as platforms. There are also white and green checkered paths around the level. There is one large tree stump with a Bob-omb Buddy that shows the location of the red coins, and a nearby platform with the Star Switch and the path to the star. At one point in the level, there is a vine that can be climbed that leads to a raised area leading to the Goomboss boss fight. The enemies in this level include Goombas, Piranha Plants, and Lakitus.

Stars
There are 2 stars in this level. There is one for collecting 8 red coins, which requires Wario to complete, as one red coin is inside of a black brick block. The second star is from pressing a Star Switch and then reaching the star, which requires a character that can wall-jump to reach.

Super Mario 64 Manifestation
In some personalized copies of Super Mario 64, Goomboss Battle appears as a special level, usually alongside Goomboss. It commonly lacks the Star Switch star due to the Personalization A.I. not recreating it, though it has been reported. Notably, a variant of it is commonly associated with the Tanooki Cap, likely it's original incarnation before it was scrapped and later brought back in Super Mario 64 DS.

Red & Blue Goomba
In Paper Mario 64, Goomboss had 2 special minions: 2 Goombas, one red, one blue, fittingly referred to as Red Goomba and Blue Goomba. They can reportedly appear in Goomboss Battle as minibosses, usually requiring 3 hits, usually jumps, to defeat them. They commonly don't have any dialogue, simply appearing, similarly to other minibosses such as Big Bullies. They usually just block important paths to the boss, but can reportedly be actually required to be defeated, spawning bridges upon defeat or dropping unique keys required to open the pipe to the boss. They can also appear in the actual Goomboss boss fight instead, replacing the Goombas following him around or having to be defeated before said Goombas spawn.