I created a custom world known as Cedola, where centuries of war between a few factions caused global devastation, with the magical equivalent of an atomic war, which only ended because a group of powerful wizards took it upon themselves to anchor one of the warring continents into another plane.
The campaign i've set takes place a few hundred years after the war's end, when the race of peoples who are sequestered into this other plane have found a way through using a new type of magic, they've staged a full-on invasion into the world, and it's up to our heroes to push back the enemy, with several twists and turns that occur throughout the campaign (including a surprise bad-guy).