a bloody rain, or a bloody downpour (see Molina)
ezmoloniliztli. sangre lluuia, o manantial de sangre. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 29v. col. 2. Thanks to Joe Campbell for providing the transcription.