The fallen angel is a monster added in dNetHack.
The angels fell.
We have only the tales of mortals to guide us on this. We,
the heirs of angels, must piece together our heritage from
the scraps of myth. We can surmise that some divine force
exiled Lucifer and his followers. We use their word, God,
because it is a convenient shorthand for our unknown
enemy. The demon race was bred to overthrow this enemy and
take its throne and whatever else we forget, we should
never forget that destiny. A full third of the Host fell,
according to some accounts. For their sins for pride,
for rebellion, for desiring to be like God, for unknowable
and ineffable reasons they were banished and they fell.
[ Infernum, Book of the Tormentor, by Gareth Hanrahan ]