Parmandur
Book-Friend, he/him
Thing is Desert of Desolation shaped the south eastern realms massively. When Old Empires came out it had to deal with the Bakar Empire and this huge desert that wasn't originally in Ed Greenwoods Forgotten Realms. This in turn lead to a radical shift in Mulhorand and Unther and possibly Chessenta from Ed's original vision for Mulhorand which was basically FR's Stygia (Conan reference) who ruled by bald Priests of Set, which got shifted to being ruled an incarnated Ra and the other Mulhorand Gods, with Set in Exile. The whole founding myth of Mulhorand and Unther and later Chessenta, and the creation of the ancient Imaskari Empire was a by product of the Desert of Desolation. It even I believe effected the History of the Durpari.
This had a massive ripple effect, from Sharess and Zandilar being merged and added to Bastet and Felidae, to changing the course of history in almost the entire Eastern Side of the Forgotten Realms. Implications even are felt as far as Shou Lung and Tu Lung, because Imaskar and Imaskari survivor state was one of the first major powers of Kara Tur, before Shou Lung.
I'd argue its one of the most important modules in FR, but weirdly indirectly, but people don't really realize the consquences of the Module on FR. The Empires it lead to were some of the biggest in FR human History, with possibly a dozen or more nations descending from them, implication across two continents.
I think the closest Module in that sort of effect on FR would be the Bloodstone Pass H series modules because they forced Ed to skrink the great glacier and add Vaasa and Damara to FR (another of my favourite FR regions), which had major implications for other regions like Impiltur and Narfell.
In FR terms, definitely: I wonder how they would work it for a new, more generic product, though.