As a shortened version of Azanulbizar, this word only shows up in drafts of The Lord of the Rings.  This is at a point where Tolkien was, tentively, explaining azan as "dim" and 'ûl as "streams".  This apparently is later changed when he interprets the -ul- of Azanulbizar as being an adjectival suffix to azan, and adding the word duban as "valley", rather than trying to fold the element "valley" into the wordAzanulbizar.  Although this form might be compared to Khuzdûl as evidence of the adjectival suffix having a long <û>, it shouldn't be since it only exists where 'ûl was "streams".  At this point, it looks like Tolkien abandoned his old interpretation of Azanulbizar and thus this word along with it.