Seal of Primus
The Seal of Primus is exactly as it sounds; 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 odd in that he is a multiversal constant, meaning he exists in all universes he inhabits simultaniously. 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 it is true 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. Also, in the DW Armada Comics Crossover World's Collide Armada Prime was drawn into a reality where Unciron had destroyed Cybertron and sent it's beaten version of Optimus Prime forward to the Armada Universe as a "calling card." If Primus was dead here, it brings up the fact that Primus could potentially die and have that particular Universe "lost" to Unicron, and so continues to exist in all other Universes, minus the ones where Unicron has been successful. Perhpas this is the true struggle between Unicron and Primus, where they fight to defend or consume all Universes on an individual level, until the Unicron Trilogy events which threatened the Multiverse. Of course, there's also the possibility of BIG GLARING PLOT HOLE, but they'll probably fix it in the flavor text.
