Orcs Must Die! Unchained takes the award-winning Orcs Must Die! action tower defense gameplay to a whole new level with team-based gameplay! But don’t get cocky, because the stakes are higher, the traps are bloodier, and the invaders are even more ravenous. You’ll need to bring your A-game and a few of the biggest orc-haters you can find!
Orcs Must Die! Unchained is set in the familiar world of its hilarious fantasy predecessors, years after the finale of Orcs Must Die! 2. With the discovery of dozens of new rifts and worlds beyond them, heroes have come from far and wide to wage all-out war for control of these magical gateways. In the years since Orcs Must Die! 2, the War Mage and Sorceress have done their best to rebuild the once-mighty Order. Their new enemy, the Unchained, is no longer the mindless horde defeated twice before, but instead an organized army led by its own powerful heroes.
Source: The Developer Website.