Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
obreptĭo, ōnis, f. [obrepo], a creeping or stealing on, a coming on suddenly or by surprise, a surprise (post-Aug.): Ventidius, aggressus per obreptionem, Front. Strat. 2, 5, 36; Arn. 5, 162: arrogari per obreptionem, Dig. 2, 4, 10: precum, Cod. Th. 5, 8, 1.
Transf. (eccl. Lat.), a surprise by sudden temptation, Ambros. in Luc. 10, § 72.