The “Unholy Roman Empire” is a medieval timeline by Midgard and G.Bone that postulates, amongst other things, Gnostic- and Cathar-influenced Reformation, an Ottoman dynasty as Christian Byzantine Emperors, alternate colonization of the Americas (known as “Avalons” in the timeline), different formation of Russia, and numerous other things.
The POD is the survival of Frederick Barbarossa and subsequent victory of the Germans in the Third Crusade, leading, through various butterflies, to stronger Imperial authority in Western Europe, and less successful Albigensian (Cathar) crusades. As a result, the Cathar religion survives, although most Cathars emigrate to Muslim Granada, giving it a much needed boost it required to survive the Christian onslaught.
Several centuries later, and in wake of an alternate Hundred Years War, where England barely managed a victory by expelling the French invaders, the Cathars form the theological core of an alternate Reformation, which later on acquires additional Gnostic characteristics. While the Byzantine (Eastern Roman) Empire enjoys a temporary resurgence under the dynasty that in OTL would have become the Ottomans, Moscow loses the battle for supremacy in Russia, which is instead won by Tver and Novgorod, only to be taken over from within by the Mongol-descended Moghul dynasty. Upon the splinters of the Holy Roman Empire, an Unholy (the idea behind the name is the Gnostic concept of an “unholy” world - hence it would mean “worldly”, as opposed to “holy” or “heavenly”) Roman Empire arises, overthrowing the Habsburg hegemony only to dissolve a mere century later into a mess of warring states.
From the war-torn Europe, scores emigrate to the New World, where they set up nations with new identities, new cultures, and new religions that mix much of the old ones into a strange amalgam. As Christianity and Islam fuse closer and closer together in an interconnected mixture of European politics, old empires fall, as the new ones rise; the dominance of Byzantium lies shattered, its old glory living only through small, if still wealthy successor states that still continue to claim the Imperium; the great Baltic League gives birth to the pragmatic, totalitarian Free State as England's experiments with democracy lead further towards the morass of chaos.
It is a strange world, the one that is both advanced and backwards, where many of the ideals that led our own world take upon a different and twisted meaning, where nations that slid into oblivion in our world still exist, and those that are known to us as powerful and lasting had not managed to arise. It is the world of the Unholy Empire and its dark legacy.
In 2006, the timeline won a Turtledove Award for “Best Medieval Period Timeline”.
The timeline can be read here.