From the official bestiary page. [http[Writings of Abd al-Hazir|writings]] of [[Abd al-Hazir]], his [[Writings_of_Abd_al-Hazir://www_Entry_no.blizzard.com/diablo3/world/bestiary/thefallen.xml_0010|10th entry - about Fallen]]:

As hard as it is to believe, the fallen ones were once exalted demons of the [[Burning Hells]]. They served as [[Azmodan]]'s hands, performing acts that he would not, could not involve himself in. They were the instrument of Azmodan's first failed attempt to usurp power from [[Diablo ]] and [[Prime Evils|his brothers]], and after that failure, the fallen were subjected to the full wrath of Diablo. They were twisted into small, ridiculous imps, in contrast to their previously powerful forms. Moreover, if they expected Azmodan to reverse their condition, they were sadly mistaken. The infuriated Azmodan held them responsible for the [[Prime Evils]]' continued reign, and so he left the fallen in their new bodies, where their degradation would serve to amuse him for all eternity. Their failure provided him with the information he required to succeed in dethroning Diablo and his brothers - the event now known as the "[[Dark Exile]]" - but that fact did nothing to soften his heart toward the fallen.