Ng’Zot Aa was one of the Cosmic Elementals who tormented Earth many years ago.
He somehow ended up at an Electronics Store called Radio Hut, where he ended up in a bowl on a pedestal.
While BMO and Neptr were on a quest to find batteries, Ng’Zot Aa started talking to Neptr, asking him if he wanted to be a hero. Neptr agreed, putting the Ng’Zot Aa-possessed batteries in BMO. BMO went mad and created a world called BMO World, where everyone could be what they wanted to be and Neptr could be a hero.
When Finn and Jake attempted to take BMO’s batteries out, Ng’Zot Aa would go on to posses Jake, making him stretch continuously until he tired out. Additionally, the world went back to normal. Later, Princess Bubblegum made Finn a Wish Staff, which Ng’Zot Aa possessed, but ended up being trapped there for weeks to come.
He then later returns with his fellow Cosmic Elementals and rains chaos upon the world, until he and the other Elementals are defeated by Penelope with the Sword of Lun and Finn. He was then erased from existance.