Dragoons appear in the Legacy of the Void campaign, representing the Aiur faction in the "ranged attacker" role.
Legacy of the Void
Compared to stalkers and adepts, the other two options for ranged attackers, dragoons have higher stats, with a stronger attack, longer range, and more HP. However, they lack the support abilities of the stalker and adept, have slower movement speed, and have a larger collision size that can hamper them in tight quarters. Their slow projectile speed and wind-up also means they have a tendency to overkill enemies, wasting shots.
Dragoons were originally in the StarCraft II game engine, but were eventually removed. While the dragoon had served as a backbone unit for the protoss in the original game, David Kim felt that the dragoon was too widely used. In his own words, "it was too good at too many things." For StarCraft II, the developers wanted to add precision "micro mechanics" to protoss gameplay. However, the dragoon didn't fit this style, as it was "a sort of all-purpose menace that just walked up and fired at enemies." Their role in the protoss tech tree was filled in standard multiplayer games by stalkers, while lore-wise the immortals are upgraded dragoons. The stalker was originally concieved as a sort of "dark dragoon."