The following section contains information from a previous version of StarCraft II which is no longer valid.
The overseer was developed for StarCraft II as the universal overlord detection in StarCraft I made the zerg "detectorific". As such, the overlord/overseer split makes cloaking more useful to a zerg's opponent.
Overseers used to have similar special abilities as the overlord, but those were removed.
The overseer can drop a non-attacking slime creature, temporarily disabling resource nodes and xel'naga towers. The slime can be attacked and removed by other forces. The minerals can be covered as they're being harvested, enabling raids which slow down an enemy's economy. The creature must be killed before the minerals can be accessed.
This ability took time to "cast" and had a small cooldown timer.
The overseer could create a nydus worm after a nydus network had been produced as of October 2008. This ability cost energy.
The overseer lost the ability to make nydus worms as of March 2009.
The overseer can create a Spore Cloud, which covers an area, blocking line of sight for a period of time. Units above the fog can see over and into it.
Overseers were to be removed from Heart of the Swarm multiplayer, due to their very limited role and lack of artistic distinction from the overlord. Its role would have been taken by the viper. However, the overseer returned, and the viper was made into a pure spellcaster while Blizzard examined how to make the overseer's abilities more interesting.
↑ 5.05.15.25.31) The tier 1 Overseer is very slow and does not have detection, though still essential for scouting throughout the game.
2) They are two different abilities. In generating creep, the Overseer essentially unloads a bunch of creep straight to the ground below it, which expands. Corrupting minerals takes some time to fully cast, but it creates a non-attacking creature that takes over that resource, and needs to be killed before those minerals are able to be gathered. Furthermore, you can corrupt resources that are already being mined, introducing potential Overseer raids to slow the enemy's production.
3)Neither abilities require energy. Creep generation does not have a cooldown whereas corrupt minerals has a small cooldown timer. Karune. 2008-04-25. Overseers having Creep Drop, details?. Battle.net StarCraft II General Discussion Forum. Accessed 2008-04-25.
↑1. No, Overseer does not have Nydus Worm spawning ability any more.
2. You can build Creep Tumor from the Queen only on the creep to expand it.
3. It just looks like other Zerg buildings while it is being summoned and the worm bursts out of the creep when the summoning is done. Hard to describe the animation of the popping up;;
4. Yes, units are coming out of the Nydus Worm/Nydus Network one-by-one and they come out in the same order they entered in. Cydra. 2009-03-2. Questions about Nydus mechanics (third post). Battle.net StarCraft II General Discussion Forum. Accessed 2009-03-02.
↑ 12.012.11) In addition to having detection and increased speed from the Overlord they can use these abilities:
Spore Cloud (50 energy)- Release a cloud of fog, covering an area and blocks line of sight for a period of time (subject to balance). Units that are above the fog can see into and over the fog.
This ability is great at blocking the line of sight of spotter units on cliffs, block line of sight of Marines while your Zerglings run up close to attack range, or even to hide your ranged units like Hydralisks from the enemy while having your own spotter for the Hydras.
Changeling (100 energy)- Changes into an enemy unit (Zealot, Zergling, or Marine) depending on the enemy race. This unit acts as an excellent scout. Karune. 2009-06-30. Karune: Can the queen heal? (10th post)Battle.net StarCraft II General Discussion Forum. Accessed 2009-06-30.