On the site Eldamo a “collection of documents is a lexicon of Tolkien’s invented languages” I just found a Tolkien Black Speech word I wasn’t aware of ‘gûldur‘ “sorcery” which is really similar to Sindarin ‘guldur’ “black arts, sorcery” as in Dol Guldur “Hill of Sorcery”. It’s obviously related to ‘gûl’ as in ‘nazgûl’. Luckely this word fits quite well into Zhâburi because one of the really old and fixed noun suffixes is -ur with a lengthened stemvowel, as in ‘zhâbur’ (language) from ‘zhab’. I already have the word ‘gûl’ (wraith (of Sauron), “any one of the major invisible servants of Sauron dominated entirely by his will”; Someone with deep knowledge of Eä from the dark perspective). So I just have to figure out why the d is there in the middle.