In the original StarCraft, the Protoss unit that can permanently cloak and attack enemy units is the Dark Templar.
Dark Templar are stealth-focused Protoss warriors. Their permanent cloaking makes them invisible to ordinary enemy units and structures, allowing them to infiltrate bases, ambush workers, and attack vulnerable targets. Opponents must use detection to reveal them before they can be targeted reliably.
Dark Templar are distinct from High Templar, who are spellcasters specializing in powerful abilities such as Psionic Storm. When two Templar combine, they can form an Archon, a powerful energy-based Protoss unit. These related names often cause confusion, but the Dark Templar are the permanently cloaked attackers.
In the original StarCraft, Dark Templar technology is tied to the Protoss technological tree and requires appropriate structures and research. Their stealth is a strategic tool rather than simple invisibility: scouting, positioning, and enemy detection determine whether a Dark Templar attack succeeds.