The Null was a powerful Subspace entity that began seeping into our universe due to the actions of a humanoid race. That race then created the Sentries to combat the Null, giving them an unbreakable directive to do so. The Null engaged the Sentries in what seemed like an endless, bitter war. Before the Sentries could be finally overwhelmed, however, the USS Titan discovered a way to defeat the Null permanently, and the Titan Avatar sacrificed herself to achieve that. (TTN novel: Synthesis)