From Halopedia, the Halo Wiki
The Stasis Cannon is a Covenant weapon found installed on the Vampire's frontal section. This is seen only thus far in Halo Wars.
When fired, it halts the targeted enemy aircraft, preventing it to escape, so that it cannot avoid incoming fire. Once the Vampire is upgraded with Stasis Drain Cannon, the weapon not only stops its target, but also drains its energy regenerating the Vampire itself and making the hostile crash onto the ground. Its second upgrade, Stasis Bomb, causes drained aircrafts to explode, inflicting Area Of Effect damage. 
- ↑ Halo Wars: Official Strategy Guide page 68,69