Marek V is a planet located within Dominion Space.
Prior to the Great War, the zerg made their presence on the world in some form, leaving behind three impaler colonies. During the Second Great War, a scouting brood re-discovered these colonies and the ancient zerg essence they contained. However, the brood came under assault by the Dominion's Prometheus Company.
In response, the Queen of Blades dispatched hydralisks to the world to obtain the essence the impaler colonies contained. They succeeded, and gained the ability to morph into impalers. They thus proceeded to eliminate the attacking terran forces.