The Third Age
With the creation of Planar’s Gate came the convergence of the planes and the resulting influx of outsiders from the various planes invaded Grayhawk. This Age is widely knows as the Dark Age since these times were some of the most chaotic and dangerous in history. The Demon Lords from the Lower Planes established footholds and cults in the world, spreading their madness and even creating a few monsterous races by mangling humanoids. Baphomet created the Minotaurs, Juiblex created his scions, the oozes, Yeenoghu created the Gnolls, and Orcus introduced the blasphemy of undead. Asmodeus, the Lord of Devils, made a pact with a coven of Night Hags to create the Yugoloths, a race of fiends for hire that plague the planes to this day. The princes of Elemental Evil established their cults and created several Temples of Elemental Evil. Titania, the Fey Queen, was finally free of the Feywild and created a bevy of mischievous minions that enchanted several forests to lead mortals into in hopes of obtaining more depraved playthings. Humans and Monsters that got to close to Shadowfell would come out as ethereal whisps of their former self, cold and dark and consuming anything that resembles life. Even the Upper planes were forced to act, creating human conduits for their divine energy they called “Clerics” to act as mediums for them to fight off the invading forces.
Other secret orders were also created to fight of these new threats. An order of slayers that called themselves “Witch Hunters” rose up to fight off fiends and undead alike. The most notable of these groups was a famed adventuring party known as the Night Stalkers went on to confront the Vampire Liege Count Strahd.
Another group that took advantage of these times were the Followers of the Sun, a cult that enacted rituals to bath themselves in the energy of the positive plane to become pure conduits of light. They persist to this day attempting to find what they call “the Prophet of the Sun” a supposedly legendary figure of Light that will one day drive away all darkness.
These times were hard and they lasted nearly a thousand years (giving birth to the phrase "a thousand years of darkness). Ending only when a Young Paladin named Heironeous would rise and bind the planes apart with his Lawful Magic in the Second Ascension.