Seal of Primus
The Seal of Primus is a seal around the life-force of Primus that keeps him in a perpetual state of slumber to prevent Unicron from ever knowing his location. Primus is a multiversal constant, meaning he exists in all universes he inhabits simultaneously. If Unicron ever found out where Primus was and destroyed him, Primus would no longer exist, meaning that his only opposition would be dead and the Matrix of Leadership would no longer work.
While Primus was "killed" in the Marvel comics adaptation, it may have just been his physical avatar in that universe, and not the collective of Primus's true consciousness, the Allspark.
In the Dreamwave Armada Comics Crossover Worlds Collide, Armada Optimus Prime was drawn into a reality where Unicron had destroyed Cybertron and sent its beaten version of Optimus Prime forward to the Armada Universe as a "calling card." If Primus was dead here, then Primus could potentially die and have that particular Universe "lost" to Unicron, yet he continues to exist in other Universes. Perhaps this is the true struggle between Unicron and Primus, where they have fought to defend or consume each universe up until the Unicron Trilogy events which threatened the Multiverse.
Of course, there's also the possibility that it's a BIG GLARING PLOT HOLE.
