Alastor is a Greek word that refers to a spirit of revenge or divine retribution. When looking for synonyms for this word, there are several choices available. Firstly, a similar term could be "avenger" or "vengeful spirit." Other options include "nemesis," "retributive force," "wrathful deity," and "divine punishment." Alternately, "just retribution," "cosmic justice," "karmic law," or "judgment" could also serve as suitable synonyms. Ultimately, the term used will depend on the context in which it is being employed, as well as the desired tone and connotation.