I always thought it was animal men.
The story (that I made up on the spot) goes that the animal men just got tired of being pushed around by the other races and banded together. They knew that even together they were no match for the other races, and that no matter where they went on the surface, the peace would be broken.
So they found a cave. The deepest and darkest one they could find so that they would not be followed. Each person packed their supplies induviduly, and marched into the cave without muttering a thing. It was a grand sight this was, all of the men of different shapes and sizes, going into this cave. A few turned back, going on to become the outcasts in the sewers and alleys of human towns.
They kept going deeper, and lived like nomads in the caves for years and years, banding together and trading with the native Troglydytes. After years of this the animal people had gone deeper than any other races, down to where they're ally's would not follow, to where the grass tasted sour.
It was then that they found the mystical metal that we all know well. They decided, all of the oldest and wisest of them, that this was the place to stay. The metal would keep them defended from anything that could attack, and the herbs and animals of the caves provided food. That is when they dug in.
And dug down.
A goatman miner is the one found it. He was digging away at the "candy", as his kin called it (that's were we get the term from), when he hit an opening. Thinking it was another cave, he came running out yelling "I found a cave!" to the administrator. He said "calm down, it's not like you found the circus, boy" as he walked down the tunnel. As he looked through the opening, he said "but pretty damn close" (that's were the term "circus" comes from). Looking back at him was a goat demon, not unlike the goatman that found the hole.
You might think that the demons would have destroyed them, but this is where that idea changes. You see, the animal men and the demons were not that different. The animal men were banished underground by the other races and the demons by Armok himself. The demons and animal men were quick friends. The animal men nutered them back to health and the demons provided the sheer firepower that the animal men were looking for. As you can imagine, a sky-blue glad swallowman is not the most intimidating thing.
The animal men, being the evil bastards that they are, wanted the demons help to take out the races that had treated them as outcasts.
But Armok was not amused. You see, when he sealed the demons away, he infected them with a plague to stop this very thing from happening. This plague took a few weeks to show any symtomes. The symtomes included rotting from the included, death and undeath, i.e zombies. This plague spread through touch, and living with demons, it broke out quickly.
Once it took notice, most of the populous left, returning to their nomad ways in caves, or returning to the surface, to think of what could have been. The elders, knowing they were being punished by Armok, huddled everyone inside, however something they didn't know was that the demons were the ones that transmitted it, and that symtomes took a few weeks to take effect.
The elders caught on to this, and decided "Seal the demons away to save them from this great death", claiming to let them out when it was over, but they were really planning to keep them there and stop the spread.
Needless to say, it didn't work. The milatary were the last ones left, living by baracading themselves on one floor, away from the zombie filled fort. However they went down one by one. The last one alive, the captain, with the best weapon in the fort, decided it was time for him to be die. He came screaming through the baracade, killing as many as he could. With his last breath, he put it weapon on the lever to free the demons, so that "only one with extreme greed, one who deserves it, will have to suffer this living hell".
DUH END