When Darkblaze took over Canterlot, he began spreading his dark energy around the world, wrapping the entire world in darkness within just a day. He trapped Princess Celestia and Luna and took the throne for himself in an effort to rule the world by destroying all hope and creating a new world order.
The Darkness itself is somewhat sentient. It can feel and react to anyone, sometimes in violent ways. The main way it attacks its enemies is indirectly, through the hostile creatures it creates, however, with magical entities, it can attack by overloading the creature with dark magic, causing massive damage. The creatures of the Darkness bear resemblance to their normal form, however they usually grow spikes or tendrils, and are always pitch black with red glowing markings. Some creatures in the Darkness are completely original, and are the most dangerous form, easily able to rip anything to shreds. Many creatures are passive, however, such as birds or rabbits and deer, which, although effected in the same was as any other animal, will still run or hide at the sight of an unknown creature. Predatorial animals are even more dangerous in the Darkness, with increased strength, speed, and endurance. The ground is always a dull, dead gray color, and the trees and plants have all withered out, normally leaving a mostly barren wasteland. A fog is suspended in the air a few inches above the ground, at an average pony's knee height, and the sky is a blood red with no clouds, covering everything in a dark red hue, decreasing visibility to normal ponies. At night it is even more dangerous, with almost zero visibility and increasing hostility from creatures. Weather in the Darkness is also much more dangerous, and are filled with dark magic.
The end of the Great Darkness came from Lightblade defeating Darkblaze at Canterlot Castle, in a battle which nearly destroyed Canterlot. Although no casualties were sustained during this battle, many were injured and even killed due to hostile creatures in the duration of the Darkness. After Light defeated Dark, the sky cleared up again, and plants and animals returned to normal. Princess Celestia and Princess Luna were found by royal guards and escorted to safety.
Much damage was sustained to cities and towns around the world, with widespread destruction all around the world. Within a few months however, with a treaty enacted by Princess Celestia, everyone worked together to rebuild, and in a short amount of time everything was back the way it was, even with some improvements. The origins of this event were kept secret from everyone, and only Light and Princess Celestia know the whole truth. Light was hospitalized and in critical condition due to his battle with Darkblaze almost killing him, from both injury and extensive use of magic. He spent a few months in the hospital completely unconscious, although he was stabilized during his later time in the hospital. After he woke up he was kept in the hospital for a few more days until he was released. The total death toll of The Great Darkness is estimated to be around 12 thousand worldwide in the original Light Requiem. The death toll in Eternal Night is over 75 million worldwide.