Spider Chair
From MYSTlore
The Spider Chair is a construction of Sirrus on Spire. The chair must first be powered up by having just the right amount of electromagnets turned on in the main palace and on the far island itself. Too many conductors online means an overload, causing a power failure, but there is a secondary power source which is not channeled through the primary power switch. The chair must then be used to reach and tune the strings on Sirrus's grand instrument. It must then be used to solve a series of frequency locks to allow Sirrus entrance to his bomb factory. Finally, it is involved in clearing a path to the age's exit by use of one of two elevators.
The reason for the spider chair's legs are unknown, they are probably just for decoration. Sirrus originally constructed the chair to play music, but later adapted it to reach his bomb factory and create explosives, although it still possesses harmonic capability.
[edit] Background
The spider chair is known as one of the most difficult and nefarious puzzles in the Myst Universe, appearing in Myst IV: Revelation.