He is a light brown, thin, mummified fish. He has black eyes, with his right eye being a glass eye, similar to Walter Haddock.
Role in episode
The mummy is first mentioned in the episode by Walter Haddock. In an attempt to keep SpongeBob, Squidward, and Patrick out of the mine, he says that there is a mummy's curse in the mine and shows a framed picture of the mummy. However, this does not scare them, and by the order of Mr. Krabs, they go into the mine anyway.
Later in the episode, when SpongeBob strikes mustard, Walter comes to scene. SpongeBob and the crew find out that the mummy's curse was made up and used as a way to keep them out of the mine. The real reason is because of the pressure build-up against the mustard, which stored under ground, erupted and sent all of the mustard to the Krusty Krab.
The mummy is used as a gag at the end, with all the mustard raining down, saying "What? No relish?"