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.”)
“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

Alaster - Dictionnaire Infernal
Reference
- DeliriumsRealm Store: Buy Alastor Merchandise
- Web: Plutarch’s Parallel Lives
- Web: Alastor or The Spirit of Solitude