This is a word that Gene Wolfe would use to describe some fantastical monster. You could tell someone that it was a minor part of the Book of the New Sun, and they'd believe you without hesitation. Alzabo, destrier,... alastor. Yep.
Usually with capital initial. An avenging demon or god. More generally: any avenger of wrongs committed; an agent of retribution. Cf. NEMESIS n. 1.