Hallucination is a sentry ability that can be researched at the cybernetics core. It allows the sentry to create “fake” units that dissipate after a while. Hallucinations don't deal any damage and have a different color scheme than regular units, but the enemy won't be able to tell the difference unless they have detection.
This ability is not as extensively used as force field or guardian shield (the other two sentry abilities) but is a great misdirection tool, a useful, if limited, damage soaker (hallucinations take 20% more damage and have no shields or armor), and produces cheap, disposable scouts.
Phoenixes are one of the game's fastest units, so a hallucinated phoenix is a very effective way to scout. You could also allow your opponent to see a few hallucinated warp prisms en route to one of his expansions, and then attack the main base while he's busy trying to destroy the hallucinations.
Mix Real with Fake
One of the best uses of this ability is to mix several fake units with your real army. The enemy can't easily tell the difference without detection, and his units will likely fire on some of your hallucinations, reducing the amount of damage sustained by your troops. Alternatively, they could misjudge the size of your forces and run away from a fight they could win... or even leave the game entirely. When creating hallucinations, remember not to create units that your opponent knows for a fact that you cannot have*.
Sentries can create the following:
- Probe (4)
- High templar, stalker, zealot (2)
- Archon, colossus, immortal, phoenix, void ray, warp prism (1)
*The sentry can create duplicates of units that you don't meet the requirements for.
This protoss player made a bunch of fake colossi, which was pretty obviously fake. Oops.