The Orcgate Wars, which began in -1076 DR, were originally a magical experiment that quickly escalated into a war of devastating proportions.
In -1081 DR, the theurgist adept Thayd, last apprentice of the Imaskari wizards, opened a fateful portal to a brutal world dominated by empires of fanatically religious orcs. After inciting the wizards of Mulhorand and Unther into rebellion, he was executed, leaving no one in Faerûn with knowledge of the existence of the portal. For five years, the portal remained dormant until the orcs began pouring through, eventually laying siege to both Mulhorand and Unther for six years.
More fanatical than those of the north, these invading gray orcs and their clerics had developed amazingly powerful and deadly spells; the most potent of magics being their ability to directly call down avatars of their deities. Avatars of the gods of Mulhorand and Unther also dwelt on Faerûn, but they had been weakened after the oppression under the Imaskari empire, being ill-prepared to defend against the orc hordes. Nevertheless, the avatars of these two clashing pantheons fought, resulting in the Battle of the Gods, a titanic clash of power that deeply scarred the land. The god Re was slain by Gruumsh with several more powers of the Untheric pantheon following, having been killed by the avatars of the orc gods. Despite the victory over their enemies, the orc armies and their deities were heavily weakened, eventually being routed by the remnants of the Mulhorandi and Untheric pantheons after a rally in -1069 DR.
So complete was the destruction of the orc horde that the gray orcs in the region have never been able to fully recover. Over the next several hundred years, they were fragmented time and time again into countless smaller tribes scattered throughout Thay, the Moonsea and neighboring regions. Constant infighting among their own kind has kept the gray orcs from returning to the power they once previously wielded.