Overlords serve the Zerg with two primary purposes. It is the swarm's transportation unit much like the Medivac once the upgrade is researched. Most importantly, they provide supplies for the Zerg minions just like the Pylons and Supply Depots. Since they are required to build a sizable force, it is common for most bases to have at least a dozen per base.
Because Overlords are an air unit available to the Zerg very early in the game, they are often sent away from the base to scout out the area. An advanced Zerg base will have enough Overlords spread out to see well beyond the base's borders. This can prove to be quite an advantage by keeping an eye on expansion points and track enemy movement.
This upgrade will enable the Overlord to carry ground units much like the Medivac. Each Overlord can carry several combinations of units including eight Zerglings, four Hydralisks, or a Ultralisk. However, if the Overlord is killed while carrying any units, those units are also lost.
Generate Creep
Requirements: Lair
Overlords are also able to create Creep below itself. While Hatcheries and Creep Tumors will still slowly expand the Creep to the maximum radius, this ability is generally used to speed up that process. Overlords cannot perform any other actions while using this ability. The Creep will only remain active while the Overlord channels this ability.
