This unit can be morphed from Zerg Overlord units. It has detector and Changeling spawning ability. In SC 2 Beta Overlords are not detector as in the past, the detector ability comes after evolving Overseer. While cancelling detector ability of the Overlord, the Overlords can create creep where the Zerg units can move faster on it. So this ability change, will also change anti-Zerg strategies especially invisible tactics like Dark Templar drop. But producing Dark Templar becomes more unefficient because of not activating High Templar Unit with constructing Dark Shrine ( It requires extra building) so the balance is protected.