Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
tĕnē̆bro, āre, v. a. [tenebrae], to make dark, to darken (post-class.): vesperā semitam tenebrante, App. M. 8, p. 208, 5; Amm. 19, 8, 5; Lact. 4, 19.