Grand Eerie

Grand Eerie is an anomalous ghostlike boss, found in some personalized copies of Super Mario 64. The details that make Grand Eerie different to an ordinary Eerie are that his eyes are red, he is much bigger than a big Eerie, he wears a crown and has sharp teeth like a Blargg from Yoshi's Island.

Boss Fight
Grand Eerie replaces the Big Boo bosses in Big Boo’s Haunt. Some Grand Eerie boss fights function similarly to the regular and big Eeries, but he has a breath of blue fire that damages Mario. When looked at, he hides his teeth and shrinks before going transparent, like a Boo. He is also damaged the same way as a Boo and Eerie would, by being ground pounded with the player's back turned towards the boss. Some Grand Eerie boss fights have Grand Eerie vanishing and teleporting behind the player once looked at. The amount of hits it takes to defeat Grand Eerie varies, with the consensus being 3-5 hits.

Balcony
This boss fight occurs on the balcony of Big Boo's Haunt replacing the Big Boo boss. The fight itself is commonly said to be the easiest of the boss fights with the player only needing to hit the boss 3 times in order to defeat it. The Grand Eerie is almost identical to the Big Boo Boss, only being slightly faster than the Big Boo itself.

First Floor
This boss fight occurs on the first floor of Big Boo’s Haunt after defeating all the Boos on the first floor. The Grand Eerie boss that appears is more difficult than the balcony Grand Eerie boss. The boss has been reported to vanish and teleport behind Mario at a close distance after being looked at. The boss only needs to be hit 4 times in order to defeat it.

Merry-go-round
This boss fight occurs on the merry-go-round after the player defeats all the Boos. The boss fight itself is the most difficult, with the player having to avoid flamethrower s, Eeries, Grand Eerie’s attacks, and Grand Eerie himself. The Grand Eerie boss can teleport and breathe blue fire similar to Bowser making it a challenge.

Theories
A common explanation on how the Grand Eerie is created is through the means of the Personalisation A.I combining the assets of Yoshi, Blargg, the Big Boo boss, and Bowser likely using the process that creates Abominations.