The headword is neuter nominative/accusative singular or masculine accusative singular -- the latter if, as is reasonable to suppose, it is drawn from the quotation given -- of prostro/paios. This adjective can refer to those who are polluted by having committed crimes requiring purification, those who seek such purification as a suppliant (as here), and the supernatural powers that take vengeance on unpurified criminals. See also pi 2829, pi 2830, pi 2832, pi 46, alpha 3297. For this connotation of the word cf. Aelius Dionysius pi68, Hesychius omicron1056, pi3900, Synagoge pi709.
[1] Quotation (transmitted, in Adler's view, via the Excerpta of Constantine Porphyrogenitus) unidentifiable. On no discernible basis, Adler suggests Aelian.