From Halopedia, the Halo wiki
Night Knight is an achievement in Halo: Spartan Strike. It is unlocked by preventing the Promethean Watchers from resurrecting a single Promethean Knight in Operation C, Mission 4. The achievement is worth 5 Gamerscore. It is represented by a circle with an Watcher inside it.
The mission requires the player to active a series of energy conduits around the map. This can make it extremely difficult to guard a defeated Knight's data purge energy signature to prevent it from being resurrected. One method is to adopt a pacifist play style: move from one conduit to another, destroying only Promethean Crawlers and Watchers while only killing Knights when left with no other alternative. Players can purchase an Overshield and/or Shield Booster to increase their durability.