adventurequest
AdventureQuest is an online browser-based RPG that started in 2002 and is still updated on a weekly basis. It's the first game made by Artix Entertainment. It is a simple turn-based monster battling game rich in content and lore.
It's major storyline is called The Devourer Saga, it tells about the return of an entity known as the Devourer to the world, with the intention of destroying the whole world.