Void rifts are NPC structures in StarCraft II: Legacy of the Void.
Void rifts appear during the missions "Into the Void" and "The Essence of Eternity". Amon's Void Shades will continually spawn from them. The player should destroy them at the earliest opportunity to avoid being overrun. 
In Co-op Missions, Void rifts appear in Korhal in the mutator "Bad Weather" of "Rifts to Korhal". Void rifts are able to spawn Shadow of the Void that can adapt the form of a random unit of Zerg, Terran, Protoss or Hybrid.