Spectres are often randomly generated in Gehennom. In addition, when a bones file is created in which the player is at least XL 15 (or XL 21 in EvilHack) and would leave behind a ghost, they instead leave behind a spectre.
Spectres are particularly dangerous, since they are likely to equip themselves with the weapons and armor that they died with.
And now the souls of the dead who had gone below came swarming
up from Erebus -- fresh brides, unmarried youths, old men
with life's long suffering behind them, tender young girls
still nursing this first anguish in their hearts, and a great
throng of warriors killed in battle, their spear-wounds gaping
yet and all their armour stained with blood. From this
multitude of souls, as they fluttered to and fro by the
trench, there came a moaning that was horrible to hear.
Panic drained the blood from my cheeks.
[ The Odyssey, (chapter Lambda), by Homer ]