You don't necessarily need supernatural creatures. Generally mythical creatures will probably suffice. For example, St. Martha's Tarasque or some sort of dragon.
But of course you don't need to necessarily settle for monsters either, or necessarily the human race as a whole. Like Bravepower said, just another group of people invading could do. You could base it on reality and have the Moors invade (although people might cry foul on account of the seemingly racist context) or you could produce an alternate reality where some Asian nation developed into a great superpower and decided to conquer the world. Medieval Europe being invaded by Chinese soldiers using 19th century weapons would be truly epic indeed.

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Original post by Portugal Stew Medieval Europe being invaded by Chinese soldiers using 19th century weapons would be truly epic indeed.

As cool as the idea is, it would be very out of character for imperial China to invade others. They already had enough internal trouble to deal with in the middle ages... Buuut since we're looking at some insane contextless conflict (machine guns and steam engines in medieval times? Ridiculous! :P) you wouldn't have to explain it away. ;)

Personally, I think aliens fighting the Knights Templar would be quite a sight. Er, the aliens would win because laser blasters beat swords however. :(

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Original post by Bravepower Personally, I think aliens fighting the Knights Templar would be quite a sight. Er, the aliens would win because laser blasters beat swords however. :(

Maybe the aliens have some sort of societal or religious taboo prohibiting sophisticated ranged weapons technology? Sort of like the nuclear arms treaties in place now, but applied to a much larger range of weaponry.

I was thinking also that there could be a giant earthquake. Then after it everyone is attacked. And at the end of the game when you go to the final area and the source of the enemy horde, you find a giant asteroid, but the characters would not know what it was because it's so long ago, and the asteroid would be home to an alien colony equivalent of a human third world country. Kind of like aliens that were still stuck using claws and teeth to fight with. I really like the idea of dragons and lizard like creatures from mythology though too. Those were some good ideas!

Going back to my ramblings on fictitious foreign superpowers, even better than chinese soldiers would be Aztec explorers conquering the "savage" lands. Just a touch of irony there.

How about this: a small area becomes diseased - the disease is uncurable - and many people die. Some people survive, marked with visible scars, while others mutated into abominations craving Human flesh. Of course they take slaves too, and oddly they are sympathetic towards people of the Islamic faith. For that reason, a great religious war ensued and Muslims believed that these mutants were divine.

...
this idea would be offensive to some. :S

A disease would be cool actually... I could probably have it come from a meteor shower or something like that. I could also have an evil supernatural creature that would be controlling the disease. Kill him you kill the disease, that sort of thing.

You could probably take some

">real life inspirations for the disease.