Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
impulsor (inp-), ōris, m. [impello], one who incites or instigates, an inciter, instigator (rare but class.; cf.: auctor, hortator, monitor): quamvis non fueris suasor et impulsor profectionis meae, approbator certe fuisti, Cic. Att. 16, 7, 2: se auctores et impulsores et socios habuisse sceleris illius eos viros, quibus, etc., id. Vatin. 10, 24; id. Prov. Cons. 8, 18; cf.: me impulsore, Ter. Ad. 4, 2, 21; id. Eun. 5, 6, 18: pravi, Tac. H. 4, 68.