The Second War was a worldwide conflict in which Herobrine destroyed most of human civilization. This is a backstory and is not part of Minecraft.
The Book of Loss describes the fall of human civilization as the Second Notchic-Herobrinian World War, but it is commonly referred to simply as the Second War.
Causes of the War
In the year 1984, Notch approved an expedition to explore the Emptiness, the barren wasteland of Orent formed during the War, in order to discover a Herobrinian stronghold and recover artifacts. The party succeeded in finding a mine and, on the night of the discovery of the stronghold, a lone miner wandered deep into it and found a shiny disc and a book in a chest. The miner decided to keep these artifacts for himself and he sold them to a wealthy librarian when he came back home after the expedition. The librarian attempted to play the disc and soon after was abducted by a party of Endermen, sent by Herobrine's associates (names still unknown). These Endermen summoned Herobrine via an ancient ritual of reincarnation developed on Indev. Herobrine invaded the librarian's body and destroyed his soul and the life forces of the Endermen, taking control of the librarian's body as his vessel.
Accumulation of Power
During the next eight years, Herobrine w on over masses of people by performing wondrous feats of "magic," such as the mining of bedrock, duplication and summoning of blocks, and various other actions many took to be more powerful than the powers of Notch. He soon came to control a third of Earth through the loyalty of fooled humans, and eventually he became emperor of a colossal state. Then, he declared war against Notch, summoning Nether creatures and the monsters of Earth, as well as a sizeable number of humans whom Herobrine had tricked into believing that he would lead them to prosperity. Notch saw this danger, and tried to reveal that this new, powerful individual was Herobrine, but Herobrine had already gained too much popularity and his followers dismissed Notch's antics. Although technology had advanced greatly in the realms of Notch, Herobrine had mastered the art of building Forms, and summoned millions of brainless hominoids to consume and infect humans. Known as Zombies, these were the main foot soldiers of Herobrine, and managed to destroy civilization because of their endless numbers.
The Undead Army
Zombies were summoned towards the end of the Seven Years's Lull, the period in which Herobrine came to power after being reincarnated. Herobrine created these undead minions in the millions and hid them away from Notch so that he would be surprised and overwhelmed by the masses of these flesh-eating monsters. Although Herobrine accumulated an army of millions of humans, creepers, Endermen, and Nether creatures, he especially preferred the use of zombies due to their relatively low value, high effectiveness, highly infective virus, and terrifying appearance in numbers. With these armies, he quickly engulfed the world in war.
The Second War progressed quickly as the undead armies enveloped the world in chaos and Herobrine's other forces subdued any resistance against their attacks.
Upon the summoning of Herobrine, Arabus lost many lands to his control as he accumulated Arabian followers. Herobrine landed on the beaches of Arabus on October 15th, 1991, only weeks after the war began. Most of the empire was overrun by Herobrine's blitzkrieg within several more weeks. The empire fell (in terms of government and unified military) in the year 1992 after Notch fled Earth due to the death of Steven during the Battle of Pointed Hill, and all signs of unification except for in the Kingdom of Jeb, currently the only surviving human civilization, vanished by 1994. It is believed by most historians that Notch may have defeated Herobrine's armies if he had not fled to his own dimension.
After the destruction of the unified Arabian Confederation in 1994, Herobrine's undead army was disbanded and the zombies were allowed to roam free across the countryside, continuing to plague what little remains of human civilization and to threaten human existence itself. Herobrine is currently the ruler of about 96% of the world and has successfully eradicated 90% of all human life. His agenda, however, demands that he does not take direct action, and instead monitors the progress of his forces from his palace in New Herbano and from Arabus. The city of Arabus itself is in a state of disrepair, although Herobrine has maintained its central districts in order to establish it as a capital of Herobrine's worldwide empire.