The Burning Legion ran rampant through the wilds of Azeroth, led by the pit lord Mannoroth and the eredar Archimonde. Malfurion and Tyrande convinced Illidan to forsake his magic and work with them to defeat the Legion. They gathered sorcerors, priestesses and other warriors together to fight the demons and free Azshara (who they believed was being held captive.) However, when Furion decided the only way to get rid of them forever was to destroy the Well of Eternity, Illidan had a hissy fit and went off to warn Azshara. Malfurion's troops attacked the Highborne immediately to prevent Illidan from spoiling the plan. It was a tough fight, but eventually Furion's forces managed to destablize Azshara's, and the whole world went kablooey and sunk into the ocean. Furion, Tyrande and their kin survived on rafts and headed west to Kalimdor.
Note: In an alternate history of the War of the Ancients -- which is now official Blizzard lore -- Illidan actually did get to Azshara and convinced her to use Neltharion's Demon Soul artifact. He planned to use the Demon Soul himself to destroy all the demons and be The Hero Of The Day, but when that plan went awry, he and Malfurion worked together to reverse a portal Sargeras had created. Illidan almost fell to the temptations of darkness, but Furion pulled him back from the edge and they were both okay. This alternate history is important because it paints a much more friendly relationship between Illidan and Malfurion than the old lore.
Anyway, this is where the alternate timelines come together. Malfurion, Tyrande and the remaining night elves set out to start a new civilization in Kalimdor. Unfortunately, they found that Illidan had stolen some vials from the Well of Eternity and had used one to create a new Well on top of Mount Hyjal. A group of elven guards tried to stop Illidan, but he killed several of them and was put on trial. Malfurion spoke in his defense, and instead of death, Illidan was sentenced to life imprisonment underground. With Illidan safely locked away, Furion and Tyrande rebuilt their society -- and fell in love -- in Ashenvale.
But the new Well was going to be a problem. Furion consulted with the various dragonflights, and they decided to plant a tree (Nordrassil) on the well and safeguard it through the Emerald Dream. All the druids agreed to enter an enchanted sleep and keep watch over the tree in the Dream. So Malfurion said goodbye to Tyrande and went down into the Stormrage Barrow Dens in Moonglade for a nice, long nap. His sleep was briefly interrupted by an attack from the remaining high elves, but he ordered their exile and went back to bed. (This is one of the many reasons why the blood elves hate the night elves.) He periodically took care of certain night elf concerns, but mostly hibernated, guarding the Dream.
With one thing and another, ten thousand years passed. Tyrande formed the female night elf warriors, the Sentinels, and waited for Malfurion to wake up. Ten thousand years without your husband? Man, that's got to be rough. Then one day Tyrande came and blew the emergency horn to wake up Furion. The orcs had killed Cenarius, the undead Scourge was rampaging through Ashenvale, Archimonde was trying to kill her, and Malfurion had left the burners on the stove on for ten thousand years. Tyrande and Furion went to wake the Druids of the Talon and the Druids of the Claw to fight the Legion once more. On their way, they fought furbolgs, spiders, and The Largest Panda Ever (seriously.) Malfurion noticed that Tyrande was a little rougher around the edges than he remembered, and she spat back that she didn't "have the luxury of sleeping through times of great peril."
Furion and Tyrande were waking the Druids of the Claw when they noticed a big elven door in the barrow den. Both recognized it as Illidan's prison, which they had totally forgotten about, the jerks. Tyrande, who was more than a little peeved at Malfurion by this point, suggested that they break in and free his brother. Furion said "No way," Tyrande said "Screw you," and Furion went back to the surface while Tyrande's Sentinels fought and freed Illidan. Illidan agreed to help the night elves (mostly because of Tyrande), but Furion refused to let bygones be bygones and would not trust his twin. Illidan -- never one of the world's great decision-makers -- decided he was going to be The Hero again, and consumed the skull of Gul'Dan so he could kill the demon lord Tichondrius. Furion banished his now-demonic brother from the forests forever. Hey, maybe if you'd come to visit sometime in the last ten thousand years, he wouldn't have gone crazy.
That night, Malfurion saw a great raven in his dreams, which told him to come to the base of Mount Hyjal with Tyrande. The couple went and met up with another unlikely duo, Jaina Proudmoore and Thrall. Everyone was about to fight when the raven revealed himself as Medivh and said they had to all work together to bla bla bla Burning Legion. The orcs, humans and night elves agreed to join up, and Furion concocted a plan. Those who recalled Furion's actions in the War of the Ancients might not be surprised to find out that his new plan involved ... blowing up Nordrassil and the Well of Eternity, again. Malfurion may be wise and patient and all those other things, but he definitely lacks a certain something in the creative planning department. Anyway, the good guys lured Archimonde -- the main Legion bad guy on Azeroth -- to the World Tree by letting him easily destroy all their bases. Then, when Archimonde was in the branches of the tree, Furion used Ancestral Guardians (aka wisps) to blow up the whole thing. Woo, the world was safe!
No, wait, it wasn't. Malfurion only had a couple of months to rest and rebuild before word came that his brother had joined up with some naga and was off fighting night elves again. Sighing, Furion and Tyrande packed up again and headed to the Broken Isles, where they helped Illidan's warden, Maiev Shadowsong, fight Illidan's troops. In the chaos, Illidan captured Tyrande and pretty much admitted that he was just trying to impress her. Remember, boys, it's generally more effective to send flowers or something than try to collapse an ancient ruin on top of a bunch of people. Illidan ran for it when Maiev and Malfurion showed up, and everyone followed Illidan to Lordaeron. Unbeknownst to Furion and Tyrande, Illidan was being blackmailed by the Burning Legion to kill the Lich King, Ner'Zhul, and was trying to use a spell to stop him. Malfurion only noticed that Illidan was doing a spell that was tearing apart the continent of Northrend.
Meanwhile, Maiev and Tyrande had taken an instant dislike to each other. When Tyrande was swept down a river in battle, Maiev quickly reported her dead to Furion. Furion went nuts and captured Illidan, breaking his spell. When Furion accused Illidan of causing Tyrande's death, Kael'Thas (who was then fighting for Maiev) piped up and told Furion she wasn't quite dead yet. The two brothers ran off to look for the woman they both loved, and eventually Illidan rescued Tyrande while Furion held back the undead forces. Grateful for the rescue, Malfurion allowed his brother to go free if he promised he'd never bother the night elves again. Illidan agreed and vanished through a portal. Then, FINALLY, Furion and Tyrande went home and took a breather.
Where he is now: Having spent his life fighting demons, orcs, his brother, and pretty much everything else in sight, Furion suffered his first real lost battle back among his own people. A group of night elves, led by Archdruid Fandral Staghelm, decided to plant another world tree so the elves would stay immortal. Malfurion warned them that it was a terrible idea, but he couldn't stick around to argue for long. The battle had drained his energy, and he had to go take a good long nap in the Emerald Dream again. While he was sleeping, Staghelm planted the new tree and effectively took over leadership of the night elves, although Tyrande is still technically their ruler.
Something has gone wrong in the Emerald Dream and Malfurion's body became separated from his spirit. He is stuck in the Dream, fighting a "new foe born of an ancient evil" called the Nightmare. Furion and Cenarius's spirit are trying to keep the Nightmare from coming to Azeroth. From the Dream, Furion also keeps watch on his brother in Outland, and suspects that things are not going well with him. Furion's body is guarded deep within the Stormrage Barrow Dens. You can visit, but I've been unable to find him so far. His condition is being kept a secret from everyone but the rulers of night elf society.
You can see Furion's spirit in Moonglade if you receive the Nightmare Engulfed Object from the four world green dragons. He also talks to Keeper Remulos about Tyrande, Illidan, and the state of the Dream. Other than that, his only "appearance" in WoW is in the Tier 2 druid set, the Stormrage Vestements.
Identifying characteristics: Big night elf druid. Purple skin, blue hair, full beard. Wears full Tier 2, as is appropriate, although occasionally he switches into the Tier 1 helm. Maybe that's the one with his ZG enchant on it? Furion is considered brave, wise and righteous, but he does have a tendency to be kind of rigid about things. Think of him like a really, really strict park ranger who will blow you up if you leave your campfire unattended.
When we'll see him again: Think soon. There's a good possibility that the Emerald Dream will be a future expansion in WoW. You can check into the dream by watching several exploration movies, one of which I'll link at the bottom. Aside from that, it's likely that Furion or Tyrande will show up when raids come to battle Illidan in the Black Temple, particularly if we end up not killing him. After all, Illidan has kept his promise and left the night elves alone ...
For more information: Malfurion on Worldofwarcraft.com, Malfurion on WoWWiki, night elf history on Blizzplanet, the Emerald Dream.