An alastor is a tormenting spirit or a Nemesis. Percy Bysshe Shelly wrote a poem (Alastor or The Spirit of Solitude – 1816), which warned idealists that if they are always searching for an ideal love, the world will be their tormentor, and they will die a lonely death.
The quote from Plutarch that de Plancy is referring to is stated below. (I don’t know if the original text actually used the term “alastor.”)
Dictionnaire Infernal – Collin de Plancy (1863) (paraphrased) Alastor is known to Zoroaster as the “executioner,” and to Origin as Azazel. In the infernal hierarchy, he is the Nemesis. The ancients called evil spirits “alastores.” Plutarch says that Cicero hated Augustus so much that he conceived of a plan to kill himself outside Augustus’ foyer, in order to become his alastor.
“Sometimes he resolved to go into Cæsar’s house privately, and there kill himself upon the altar of his household gods, to bring divine vengeance upon him; but the fear of torture put him off this course.”
– verse 68 of Cicero’s bio in Plutarch’s Parallel Lives