The Story of Illidan Stormrage
Great harm can come from good intentions, and Illidan is a great example of this. Throughout his story, he keeps trying to help save everyone, but he makes too many sacrifices to achieve his goals. The idea that "the end justifies the means" only works some of the time. It's also a story about why magic is inherently bad in Azeroth. It all starts back at the War of the Ancients. There are several accounts of this story and different timelines. Essentially, they break down like so:
In Warcraft III, Illidan betrays Malfurion and warns Azshara that Malfurion plans on destroying the Well of Eternity. In this scenario, he steals several vials of the well's water as Malfurion and Tyrande storm the palace and destroy the portal. In the books, it happens differently. First, Malfurion never plans on destroying the portal. Illidan is one of the mages who helps defend the Night Elves until he starts getting jealous of his brother and Tyrande. In this case, he starts killing Night Elves as he casts massive spells to destroy large groups of demons and generally starts going crazy. Eventually, he betrays the Night Elves and joins Azshara. His eyes get burned out so that he can see the flow of magic around him, and he sets out to steal the Demon Soul. He succeeds by stealing it from Malfurion, who stole it from Deathwing (makes sense... right?). He returns with it and gives it over. Again, feeling jealous and that people were trying to cast him aside, he turns on Sargeras, who wants to wield the Demon Soul instead of Illidan (due to trust issues, obviously). Illidan breaks Tyrande out of jail and then starts casting a spell to close the portal. Of course, the Old Gods, sensing a chance to break into the world, start manipulating him and his spell. His spell doesn't work. Malfurion shows up, and Illidan is surrounded. Malfurion, who has the Demon Soul again, uses it to battle and eventually close the portal (while fighting off the Old God's influence). When the portal closes, it accidentally blows up the portal and cuts Sargeras in half, who was halfway through the portal. In both accounts, at the end, Illidan is locked away underground indefinitely. In both accounts, he also steals several vials of the Well of Eternity, but they differ greatly in his intentions. No matter, the story can easily be interpreted as him swearing allegiance to the Burning Crusade at some point.
Things start to come together once you reach the Warcraft III part. He's let out of jail by Tyrande, who thinks he can help battle the Legion (since he did it once before). He agrees, since he still loves Tyrande. Eventually, he ends up in Felwood where he fights Arthas. They duel and come to a tie, at which point Arthas tells him about the Skull of Gul'dan. Illidan, being gullible, went and obtained it, then consumed it, which brought about his demon form. He then kills Tichondrius for one of the several times that guy dies in the story. Once Malfurion and Tyrande see his demonic form, they feel disappointed in him, which drives him a bit crazier. They also let him go again for like the 100th time. Kil'jaeden shows up and offers him a chance to serve the Legion again as a way to try to destroy the Lich King before he becomes a problem for the Legion. Illidan accepts and heads out to melt Northrend. First, though, he needed the Eye of Sargeras. So he goes to the Tomb of Sargeras, where he retrieves the eye and summons the Naga to his side at some point. The warden Maiev Shadowsong chases him here and attempts to arrest him. She fails and has to flee after losing most of her party in the destruction of the tomb. He then flees to Dalaran once Malfurion and a ton of reinforcements show up. At Dalaran, he begins to use the eye in order to melt Northrend and destroy the Frozen Throne. He fails when Malfurion arrives, along with the warden and Prince Kael'thas. Malfurion is angry that he's trying to harm the world while Illidan argues he's doing it for the greater good. Again, great harm can come from good actions. This is a common theme in the story of Illidan.
Lore Theorycrafting
This is where you, the members of our community, come in. I'm going to give a fictional lore scenario and what I think would happen, and you can hash out your thoughts on the forums by clicking here and replying to our thread. So this week, let's look at this scenario. Let's say that Illidan succeeds in melting Northrend and destroying The Lich King. What would happen?
Personally, I feel that the Alliance would have eradicated the Horde after most of the coastline was lost to the flooding that would ensue. The Night Elves would have lost most of their land to the water, but of course, Hyjal would remain. The Burning Legion, at a later point, would come up with a plot to return, and Azeroth would fall without the combined forces of the Alliance and Horde.