The newest DLC pack for Mass Effect 2 features a rougue, sentient AI to battle wits with and adds five new planets to the Galactic map, each of which can be explored with the Hammerhead tank that was first featured in the previous Firewalker DLC pack. Bioware touts this expansion as the most sweeping of the contest released yet for ME2 and has been speaking about the fact that players should expect a horror-type experience complete with flickering lights, dark corners and a perturbed alien intelligence pulling the strings and playing with all the switches.
Overlord runs a little over two hours and features some seriously upgraded handling on the improved Hammerhead vehicle. For extended details, have a look at the GameSpot interview with Mass Effect 2 executive producer Casey Hudson. This DLC pack should drop sometime in June.