The Mongolian Fireball is a special and difficult kung fu technique, as it is one of the Twelve Impossible Moves. It was first seen in use by Fenghuang in the episode "Owl Be Back" from Kung Fu Panda: Legends of Awesomeness in demonstration to Po, admiring his ability to quickly learn and use difficult techniques.
The Mongolian Fireball is a technique where the user goes into multiple stances, and then brings their hand close together vertically. After this, a large fireball similar to the sun (only smaller) forms between their hands and slowly moves forward and acts like a black hole by sucking nearby objects in. The fireball then gets shorter until it disappears, leaving only a cloud of dust behind.
The Mongolian Fireball was first used in "Owl Be Back". After Po exiled himself to prevent harm to the Furious Five and the Valley, he discovered Fenghuang's hideout. After some convincing, Po refused to turn evil. He then chased Fenghuang back to the Jade Palace, where he was then taught such impossible moves as the Mongolian Fireball.