Here's another baddie that managed to get their own expansion. Sort of. See, the Lich King isn't so much a villain unto himself as he is a really nasty, spooky helmet. It can talk, though, so that probably qualifies it for full character status. Hell, a baby murloc is a champion in Blizzard's Heroes of the Storm, so clearly they'll take anyone these days.
Anyway, back in the much simpler days, when Warcraft was known as a strategy series to people under 30, there was this orc named Ner'zhul. By "warlord and conqueror" standards, he was pretty alright. He united the disparate orc tribes, saved his race from total demonic possession, and, when it was revealed he had unknowingly been working for demons in the first place, told them to piss off. So far, so good.
Unfortunately, not everyone shared Ner'zhul's vision of a free orc society. His apprentice Gul'dan, another big bad in Warcraft lore, betrayed him to the demons. Twice. The first time only got Ner'zhul stripped of his power in the Horde. The second, however, drove him insane, power mad, and right into a demonic trap that got him torn into itty bitty green chunks.
What does any of this have to do with the Lich King? Well, there's death in the Warcraft universe, and then there's "death" in the Warcraft universe. While Ner'zhul's body was destroyed, his spirit was trapped inside a block of ice -- like the most poorly cared for goldfish in history. That is, if a frozen goldfish also got infinite, godlike power over life and death.
Ner'zul, now the Lich King, was already sort of evil at this point, what with going insane and all, so the addition of more demon-infused power really did him and the rest of Azeroth no favors. Anyone who comes into contact with his disembodied headwear gives the Lich King physical form, and is forever cursed to never, ever shut up about it.
The orcs nation has gone through a lot in the Warcraft universe. They've been invaders, sure, but they've also been beaten, corrupted, and enslaved by just about every other major power they've come across. If it weren't for the healthy dose of time travel shenanigans involved, I'd hesitate to call Grom Hellscream a villain at all. Instead, he'd seem more like a very angry hero.
His multiple rises and falls from grace are also pretty well-known. They were mostly chronicled in Warcraft 3 and World of Warcraft: Warlords of Draenor. Unlike well-known traitors like Arthas and Illidan, however, Grom deserves special attention because of just how complicated his relationship with the factions of Azeroth became.
Grom kicked things off pretty innocuously. He was a chief among orcs during the far simpler time when humans were their biggest (visible) concern. After getting kicked in the teeth and stranded on Azeroth, Grom spent a good long while waiting to die. That is until he met fresh-faced young shaman called Thrall, son of Durotan, long-time leader of the Horde and all-around snappy dresser.
After a quick and bloody conflict where they freed the rest of their kind from internment camps, the pair sought a new home where they could live in peace. Unfortunately, that led them into contact with the very same demons that had gotten the orcs into their current predicament in the first place. Grom, clearly not well-versed in his people's history, took a swig of a demon's blood and once again became a literal thrall to the orcs' old, dark masters. Thrall-the-person wasn't having any of this. He worked to free his old friend of the renewed curse. They succeeded just long enough for Grom to redeem himself by sacrificing his life to kill the demon that cursed him.
It was a happy ending for everyone. The demon was dead, Grom died a hero before he could drink any other suspicious fluids, and Thrall didn't need to have an awkward talk about which of them got to be leader of the Horde.
Nope. Grom's son, Garrosh, had to ruin things by going back in time to stop the orcs from ever being enslaved by demons in the first place. That seems like a well-intentioned plan, except that without the calming presence of Thrall an alternate, angrier-than-ever Grom Hellscream led a brand-new Horde from the past to kill and subjugate everybody else. It just goes to show that, even in fantasy, children ruin everything.