black wizard

[Lord of Angmar]

I know. As I said, both were Maiar, but Balrogs were not Istari and Istari were not Balrogs. Therefore it can be said that though both were Maiar, they were subdivided into different groups. The Istari were subdivided by their physical appearance and their purpose for entry to Middle Earth, and the Balrogs grouped by the fact that they were evil spirits who had served Morgoth and who could no longer change from their forms.

but there were no specific "groups" of Maiar

As I said above, yes, there were. Just like there are different groups of orcs (Uruk-hai, snaga, etc.), different groups of Elves and Men (Sindar vs. Noldor, Gondorians vs. Haradrim, etc.) and just like in real life almost every species or race can be broken down into sets of subdivisions, so too could some of the Maiar be grouped into different categories.

[ October 19, 2003: Message edited by: Lord of Angmar ]