Here's what MXR say about the M103 Blue Box Fuzz Octave:
This is the crazy effect heard on Jimmy Page's "Fool in the Rain" solo. The Blue Box Octave Fuzz takes your guitar signal and duplicates it two octaves down for a huge, subterranean tone. The Blend knob lets you control how much of the fuzz tone is mixed with the two octaves down signal. Not for the timid—there is truly nothing like the Blue Box.
This pedal can be powered by a 9-volt battery, a Dunlop ECB003 9-volt adapter, or the DC Brick, Iso-Brick, and Mini Iso-Brick power supplies (not included).