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. Of course, there's also the possibility of BIG GLARING PLOT HOLE, but they'll probably fix it in the flavor text.

From Teletraan I: The Transformers Wiki, a Wikia wiki.