
.
It was like the original RPG (obviously not the actual original one) but it was one of the first to be hugely popular. It was the game that won Blizzard much of its popularity. It had everything; and although the graphics are now outdated it provided the basis for most of the games of the 'hack and slash' genre that followed.
And as my explanation is probably not good enough I'll resort to good old Wikipedia.
DiabloDiablo IIDiablo II holds (or at least did hold at the time) the record for the fastest selling computer game ever sold. 1 Million units within 2 weeks.