Orcgate Wars
The Orcgate Wars were a major apocalyptic event in the history of the Forgotten Realms setting of Dungeons & Dragons, resulting in the devastation of the nations of Unther and Mulhorand.
Now, orcs had existed in the Realms for a long time, inhabiting the cold mountainous regions of northern Toril. These "mountain orcs", as they are now called, had never really been much of a threat, all things considered, being largely cave-dwelling brutes. The source of the Orcgate Wars lay instead in a race called "gray orcs"; more technologically advanced, deeply religious and with access to powerful divine magic. Natives of a world far removed from Toril, the gray orcs invaded Toril in the year -1081 Dale Reckoning as part of the vengeance of a powerful Mulan wizard named Thayd.
See, Thayd had studied arcane magic under the Imaskari, and despite being a Mulan, wasn't too thrilled with the priest-ruled status quo of Unther and Mulhorand, which saw wizards pushed to the side. Having discovered the world of the gray orcs with his planar magics, he created a hidden portal to their world as a backup plan, before trying to incite a wizardly rebellion against the Priest-Kings of the Old Empire. It failed, he was captured, and before they executed him, he promised them that they'd regret it.
Sure enough, several years later in -1076 Dale Reckoning, the gray orcs found the portal on their world and launched an invasion onto Toril. Despite being backed by their gods, the Mulan found themselves sorely pressed, because the gray orcs could summon their brutish and savage gods to fight at their side too. Thus, the Orcgate Wars, which saw the deaths of most of the Untheric pantheon and several of the Mulhorandi gods before the gray orcs were finally broken and scattered, leaving Unther and Mulhorand in ruins and sliding inexorably into decline.
Incidentally, Thay considers Thayd their spiritual precursor, and honor him as setting events in motion that allowed for their nation to ultimately assume power.