The Mu Spirit is a character in EarthBound. It appears during Poo's Mu training and is said to be a spirit of Poo's ancestor.
When Poo climbs to the top of the mountain and begins his Mu training and after ignoring distractions, a visual transition occurs, making everything but Poo dark, probably to show that he has stopped focusing on his surroundings. Shortly after, the Mu spirit appears as the head of a ghost-like person, claiming that he is the spirit of Poo's ancient lineage. He asks if Poo is okay with breaking his body parts, and if Poo agreed he begins demolishing his body, which also leads to Poo's HP decreasing for every part that he breaks. If Poo ever would say "no", the spirit leaves and Poo fails with the trial and must restart the entire procedure.
The Mu spirit will start with breaking Poo's legs, followed by tearing his arms off, then cutting his ears off, and lastly take his eyes. He then mentions how Poo only has his mind left, which he eventually will take as well. After that, Poo returns to reality, and is told that he has completed the final trial of his training and must return to his master in the palace.