You have a group of immigrants fleeing their world which they destroyed with devil worship to a world which they are also bound to destroy in the same way. Some of them are decent people, but a lot of them are corrupted by this foul magic and it manifests in their eyes glowing green, the traditional color of Islam. The world they're invading is ruled by a close alliance of countries. They didn't come there on their own, they were invited in by a powerful figure who was supposed to protect this alliance.
You seriously can't see parallels with the way right wing parties are painting the refugee crisis?
This is one of the most retarded stretches I have ever read
Gul'dan turns Draenor into a barren land with fel magic. Fel magic offers great power at the expense of feeding on life, the first Orcish warlord Gul'dan uses draenei prisoners to open the great gate and facilitate more conquest. Amongst them are warriors who do not want to take part in Gul'dan's dark magic, but join his warhost anyways cos the promise of a new world is quite alluring when the choice is remain in a barren land or move to greener pastures. Amongst them are the Frostwolf clan Durotan and his mate the pregnant Draka. Draka is forced into premature labour as a result of passing through the gate and their child is born stillborn, Gul'dan harvests the life energy of a nearby deer and saves the child's life with his fel magic, indebting Durotan and turning the baby from Orcish green to fel green.
The human general Anduin Lothar visits the Dwarven King Magni Bronzebeard upon hearing of an attack on some outpost garrisons. Some young mage tells Lothar (at this point investigating what is going on) telling him the fel is at work, whilst not telling him what the fel is. The plot thickens when Lothar goes to an old mage friend of his called Medivh, encountering him whilst Medivh is squabbling with a younger and ambitious mage called Khadgar. There there is exposition where Medivh says fel magic offers immense power at terrible cost, feeding directly off of life energies. Given this is after we've seen mass sacrifice to start interdimensional invasion or bring back stillborns to life, it's pretty damn powerful, at pretty high cost.
The three get ambushed by corrupted fel orcs, who end up killing most of their entourage, until Medivh casts some spell that kills all the corrupted Orcs. The Orc we were introduced to earlier Durotan is one of the few Orcs not slain, as he was not using fel magic, who promptly flee the scene. As Durotan is fleeing, he spots one of Gul'dan's slaves Garona chained up, and after hesitation cuts her chains. Garona is especially important because she is a half-orc half-human who can speak both their languages, bridging the two (indeed, that is her character motivation). Garona is captured by Khadgar, they are brought to Azeroth, and Queen Taria wins her trust and her help. Garona also explains who the Orcs are, and how since their world is dead, all will be invading Azeroth.
Meanwhile back at Orc camp, Gul'dan blames warchief Blackhand for the failure of the ambush, and sentences him to death. Blackhand accepts the punishment, but Durotan says fuck this shit, fel magic caused the ambush to fail. Durotan severs Blackhand's arm, saving his life, causing a schism in the horde ranks. Durotan's second in command Ogrim Doomhammer converse and realize that the corruption that destroyed their home is being spread by Gul'dan, Durotan decides that the only way to secure a future for his family, his clan and the orc people is to overthrow Gul'dan. Realizing that their clan alone is too small to defeat Gul'dan's forces, Durotan proposes an alliance with the humans.
Lothar, Khadgar and Garona travel to the Horde encampment, having some character development and shit. Then they get ambushed by Durotan, who just wants to talk. Durotan requests a meeting. Meanwhile King Llane tries to call them all to fight, but due to council infighting, only Stormwind goes to war. King Llane agrees to the meeting request upon receiving it from Durotan, accepting the vouchsafe from Garona.
This sets about the mechanisms for the finale which lead up to quite a surprisingly good ending, establishing the lore behind Alliance and Horde.