Deus ex Machina: An ancient cabal of mortals who have been denied godhood at every turn begin construction of a machine that, powered by the souls of countless innocents, will destroy the multiverse and recreate it with them as the masters. A Planescape game (loosely) based on Tad William's Otherland.
The Masters: A hero of legend, famous for his defeat of 10,000 orcs with only a 1,000 men, has emerges from magical suspension with an army of angelic beings. In an Aurthur-esque "returning at the time of greatest need" fashion, an army of orcs invades simultaneously from the west. Unfortunately, the returned hero is a fervent believer in humans as the master race. If he wins the war, he will begin to push deeper into the orc territory, killing every orc that he can find. Eventually his hatred will spread to other races, including elves, dwarves, gnomes, etc. When will the morality of the heroes force them to turn on the "great hero" and can they stop the orcs without his army? And, since his angles are made using positive energy infused in the bodies of his fallen foes, will they be able to stop his army if they try?
Power Corrupts: On an extremely low magic island loosely based on post-roman britain, shortly after an apocolypse that resulted in the occupation of Scotland by a massive undead host that constantly assaults Hadrian's Wall, a new power begins to rise. A small rift opens to a power of infinite evil and the heroes must race against multiple foes, each more powerful than the last, to seal the rift before its power can be used to conquer the world. However, when they arrive at the rift they discover that they have two options: (1) seal it for the next thousand years or (2) bond with its infinite power and use it for
good to defeat evil forever. In a world where few can afford to eat, let alone forge magical items, which do the adventurers choose? Power or peace? And remember...power corrupts.*
* Bonus plot: If they choose to bond, the next campaign is to overthrow the evil overlords of the island who are bonded with a rift that grants them infinite power
.