The Osiris Distortion/Sustainer is adapted from the Black Arts Toneworks Pharaoh Fuzz, a Big Muff-based circuit with a few modifications.
First, an input impedance switch has been added to the front of the circuit. By reducing the signal level before the boost and clipping stages, you can match it to high-output instruments or earlier pedals in the chain, or just reduce the overall signal level for more headroom and less clipping.
Second, a clipping switch has been added to control has been added to go between silicon and germanium diodes in the second clipping stage. The silicon diodes are in the stock Big Muff for a familiar sound, while germaniums will be more compressed and distorted, but also softer with a more rounded clipping waveform.
Third, a “High” control has been added to the bass side of the tone control rotation allowing the low-pass filter frequency to be adjusted. This restores some of the high end so you don’t have to choose between a bass emphasis and treble emphasis.