Gamebryo Engine

Gamebryo is a commercial engine best known from single-player titles (the Elder Scrolls and Fallout series), but it's also been widely licensed for MMOs — from Dark Age of Camelot to MapleStory 2 and a number of Korean titles. The engine's flexibility meant studios heavily modified it for their own needs, which is why Gamebryo implementations across different games often look quite different from one another.