MERCILESS formed with original vocalist Kalle in 1986. The group released two demos, 'Behind The Black Door' and 'Realm Of The Dark'. As drummer Stefan Karlsson joined Punk act DIA PSALMA was superseded by Peter Stjärnvind in 1991. MERCILESS released a promo split single with COMECON the same year.

Various members of the band pursue careers in other acts. Stjärnvind is also a member of UNANIMATED, FACEDOWN and Punk outfit LOUDPIPES. Bassist Fredrik Karlen is also in LOUDPIPES rebilling himself Carl Leen. In their time the group had opened for SEPULTURA, ENTOMBED and SODOM. Band drummer 'Stipen' would join TRANSPORT LEAGUE during November 2001, subsequently enrolling into BLACKSHINE.

MERCILESS reformed for an eponymous 2003 album. Their Stockholm show in August of that year would be captured on the DVD 'Live Obsession', this also including video from a 1999 performance. Both Erik Wallin and Stipen subsequently joined HARMS WAY.