Ulrezaj had stolen a sacred khaydarin crystal from Shakuras, and while tracking it, the protoss discovered that Ulrezaj had created five twisted khaydarin crystals on Char. He was using them to control zerg minions.
Zeratul needed a dark archon to destroy the warped crystals, as only its psychic fury was capable of it. This Dark Archon Hero was accompanied by the High Templar Hero as well as several other heroes.
The high templar selects a unit, creating two duplicates of it. Hallucinations appear with a blue tint to the creating and controlling player, while all other players will see them as duplicates of the original. Hallucinations do not inflict damage when they "attack", may not cast spells, or produce units (important for hallucinated carriers and reavers), and take double damage from all attacks. Hallucinations last for a limited time and automatically disappear when hit by spells (eg. EMP shockwave, Mind Control).