The Valkyrie is a Terran air-to-air unit that is available to be built from a Starport once both an Armory and a Control Tower are on the field. Valkyries employ an air-to-air attack that issues a volley of eight small missiles at its target. These missiles impact not only on their target, but also damage any enemy units within a 3 x 3 matrix area. Each missile does a small amount of splash damage in this target area.
Once Valkyries shoot their first missile, they are unable to move until all eight have been fired. This makes them vulnerable when in small numbers so they require support (Marines, Goliaths or Missile Turrets).
In Patch 1.18, a bug was fixed that now allows Valkyries to attack appropriately. Prior to 1.18, Valkyries would not fire if there was a lot of activity on the map - this included large numbers of units, intense fighting, or both. It appeared to be related to the number of sprites being rendered by the game engine which affected all units; once that cap was reached, some units wouldn't fire. However, for whatever reason, the Valkyrie was always the first unit to be affected by this now-fixed problem. This was a huge problem in a non 1-on-1 setting as this cap had prevented Valkyries from being used when they would be most valuable - when there are large groups of air units. The Valkyrie now works correctly without problems.
The use of Valkyries in TvZ was very rare until Fantasy introduced his variance of the Mech TvZ build in the form of the Fantasy Build. The use of Valkyries by Terrans dramatically increased, and for a time the Metagame for TvZ commonly involved the use of Valkyries to defend against Mutalisk harassment, although this style has become a bit more uncommon as of late.
Valkyries have been seeing a great increase in competitive usage recently. The current Metagame involves Terran using a bio opening into a mid-late game mech switch, which Zerg like to mitigate with late game Mutalisk switches or mass Queens to cast Spawn Broodling on Siege Tanks. Since Terran players typically have two Starports with Control Towers for Science Vessels, adding in some late game Valkyries to help deal with Zerg threats from the air is convenient and can be quite effective.
The Valkyrie is used occasionally to combat cloaked Wraiths, since the splash damage will harm the cloaked Wraiths if it is in the splash radius. Even adding a single or small group of Valkyries can pay dividends in big Wraith versus Wraith engagements.
Valkyries have no known use against Protoss. Protoss traditionally only use Carriers, Arbiters and Shuttles against Terran. Carriers have very high armor making Valkyries ineffective in combatting them, and Arbiters and Shuttles almost never stack in large numbers, in addition to Shuttles being too fast and Arbiters also having too much survivability and speed for Valkyries to be much effective.
Because Valkyrie can not usually move while firing its lengthy eight missiles, they are often vulnerable to Scourge. Because of this it is important to keep Valkyries close to a Marine and Medic army in order to protect against such sniping. A trick that can be used to help accomplish this is as follows:
- Move the a Valkyrie (or stacked group of Valkyries) towards the intended target.
- Immediately patrol in the opposite direction
The effect should be that the Valkyries acquire a small amount of momentum in the direction patrolled slightly before firing its missiles if it does at all. This movement is not only something that is useful against units already in range, but also to spam towards potential engaging Mutalisks or Scourge. Utilizing this movement, any Mutalisk that comes in range of the group of Valkyrie as it patrols back will be showered with all eight of every Valkyries' shot, regardless of how far it runs. An example where such a technique and it's benefits become evident is the Fantasy vs. Type-b under Notable Games. Fantasy repeatedly uses the move forward-patrol back motion and often punishes many of Type-b's Mutalisks for coming too close.
While the exact positions that each of the Valkyrie's missile land seem random, this is not the case. Each Valkyrie cycles through exactly 14 positions for every rocket fired. Out of these, 1 hits dead center, 4 strikes within a distance of 15-20 pixels, while 9 land off-position by 50-60 pixels. On average, you can expect a Valkyrie to hit the target ~36% of the time, while dealing only 50%/25% splash damage ~64% of the time.