The Orcs were once a shamanistic race. Until a powerful Demonic army known as the Buning Legion came and corrupted the orcs. They turned them into bloodthirsty beasts and used them to attack the Human nation of Azeroth. No one knows where these fierce creatures came from, they had green skin and unrivaled savageness. Attacking in overwhelming numbers, they quickly became known as 'The Orcish Horde'.
(continuation from Elves) The Orcs entered Azeroth, and the Humans were physically much weaker than the Orcs, so the Orcs thought there would be no problem in defeating them. They attacked Stormwind Keep, the heart of Azeroth. However, despite their power, they relied too heavily on brute strength and lacked strategy. They were soon beaten back. In this assault, King Wrynn III was killed, and Llane became the new king. The war raged on for about a year. During that time, Garona, a half-orc and assistant to Gul'dan (an Orcish warlock), managed to sneak into Stormwind Keep and kill King Llane. On the Orcish side, WarChief Blackhand was also slain, however he was killed by another orc, Ogrim Doomhammer. Gul'dan had been using Blackhand (as he was easy to manipulate), and now his plans would have to be rethought. Ogrim Doomhammer became the new Leader of the Horde.
Gul'dan
Meanwhile, Medivh was killed by an Azerothian raiding party in his tower. Gul'dan managed to gain the location of the Tomb of Sargeras from his mind before he died, but was put into a coma in the process.
At the end of the war, the Orcs managed to capture Stormwind Keep and drive the humans out. Lothar (a human paladin), acting as the temporary king of Azeroth (Llane had a son, but he was too young), exiled his people across the Great Sea to the lands of Lordaeron.
Lothar met with the King of Lorderon, as well as leaders of the other 5 nations. An Alliance was soon formed, these nations used to be part of the Nations of Arador, but broke apart.
Meanwhile, the Orcish Horde began to mobilize their forces across the great sea for their final strike at the Alliance.
And at last, after many years Gul'dan has finally managed to find the Tomb of Sargeras and he freed the Daemonlord. Gul'dan was later killed by Ogrim Doomhammer, however.
The Daemonlord Sargeras
During this war, in fact around the start of it, The Orcs allied with the Trolls after rescuing one of their leaders, Zuljin. The Orcs also allied with the two-headed Ogres. The Humans eventually did win the war however, reclaiming Azeroth from the Orcs, and destroying the Dark Portal which brought the Orcs to Azeroth. But to the surprise of the Alliance, although the Portal was destroyed, the rift that connected the two warring worlds yet remained...
Both sides were badly crippled and all of the remaining orcs were hunted and placed into prison camps. The nations fought over what to do with them, and without a common enemy, the Alliance soon began to break apart. Meanwhile, The Orc Warlord, Kilrogg, of the Bleeding Hollow Clan escaped back through the rift to Draenor. Ner'zhul, the current leader of the Clans of Draenor, met with him, and a plan was formed to gather artifacts so that the Orcs could open their own portals to the Twisting Nether. Ner'zhul and his band of Orcs returned to Azeroth, gathering the artifacts. Meanwhile, the Humans entered Draenor to stop the orcs once and for all. In the end, Ner'zhul got the artifacts, and created several portals. However, the portals started to destroy and tear apart Draenor. Being the coward that he is, he entered the portal leaving the Orcs to die in Draenor. The Human heroes following him and sealed the Portal to Draenor (so the destruction did not consume Azeroth as well), and entered into one of Ner'zhul's portals to survive.
Meanwhile, the orcs were stuck in Prison Camps on Azeroth. Thrall, a young Orc, was growing up inside a castle owned by Lieutenant Blackmoore. The young orc was found on a battlefiend during the First Great war, and raised by Blackmoore as the ultimate killing machine. Not a lot is known about the exact story, but Thrall escaped, and learnt about the Orc's heritage, and their original Shamanistic practices before the Legion's influence. He freed the Orcs from the slave camps and rebuilt the Orcish Horde, this time without the influence of the Demons. He became the new Leader of the Horde and established an elite group of Orc shamans known as Farseers. Farseers could foresee the future and could control and conjure fierce thunderstorms, these orcs, Thrall trusted without question.
While wandering the Twisting Nether, Ner'zhul was found by the Burning Legion. Either by choice or force, he was
formed into an Undead Lich King, and now commands the Undead armies of the Scourge. This was the
power he was so desperately seeking, and now he seeks vengeance. Ammassing his undead army in the frozen Northern Continent, he prepares for his assault on Azeroth.
Meanwhile, the Burning Legion is ready to attack. Azeroth is weak, Sargeras is free, and the time is right, the third Great War has begun.