Euronymous

Due to the years of him being alive and to the unmistakable look of this pedal, the photo clearly shows proof of him using an HM-2.

Without any shadow of doubt, the pedal boosting the HM-2 in the attached photo is a BOSS SD-1 Super-Overdrive (obviously Made in Japan) because the BOSS OD-1 had two knobs only, the OD-2 had four knobs and the weren't any other yellow BOSS pedals around at that time (except for a wah, but Euronymous seemingly never used one). In-fact if you follow the cables in the photograph - you can see the SD-1 is stacked into (placed before) the HM-2, this would create more compression/distortion in the signal instead of volume boost.

Euronymous played a sunburst Gibson Les Paul guitar, which can be seen in many pictures of him playing. He played through a Marshall Half Stack and used an Ibanez Tube Screamer pedal as well as another distortion pedal to create a unique sound.

This photo by a facebook page dedicated to Mayhem collectibles, shows the Boss DD3 pedal that Euronymous used when recording the track Freezing Moon

Euronymous used the Arion SMM-1 Metal Master distortion pedal in his setup, placing it before an Ibanez Tube Screamer and after a Marshall Plexi to craft his distinctive black metal guitar tone, as noted on eBay.