According to
Genesis 6:4, the Nephilim or
Giants are the offspring of the 'Sons of
God' (angels) and mortal women.
Rabbinic wisdom has it that the
Flood was sent to kill the
Nephilim, whilst their angelic progenitors were cast into the chasm of
Dudael (according to
Ethiopic Enoch). There's no reference to the Flood being directed at
Seraphim in general, who include many loyal
angels and demons. The Nephilim and their fathers may be identified with the followers of the tattered flag in
Dante's
Inferno,
canto 3, in the
Vestibule of
Hell.
2501 needs to tighten the definition of
Apocrypha, by the way. The books of
Enoch are
apocryphal (and
pseudonymous), but are not considered canonical by
Protestant,
Reformed,
Catholic or
Orthodox Christians, or by any major group of
Muslims or
Jews.