There are a lot of things about TERA that have helped push it to the forefront of people's minds without having seen much of the game, but the lore is certainly a big part of it. Aside from being a fine way to wave away fantasy inconsistencies, there's something infinitely unique about a world being dreamed into existence. But life in Northern Shara (with Shara being the eastern continent) is anything but a peaceful dream, as it's the site where the unnatural Argon first burst into the world. A new lore entry on the official site goes into detail about just what is happening in the besieged environment.
Argon forces are both the core threat of TERA and the biggest danger to the region, but they're not after simple conquest. They've imported vast machinery to help them terraform the world into a more fitting environment, crawling across the landscape and distorting it. Still, within the city of Kaiator, the Valkyon Federation (presumably including the player characters) works to stem the flow of invaders and disrupt their efforts. Take a look at the full description for a better picture of the world's first battlefield against what seems to be utter annihilation.