Seal of Primus
Eons ago, due to the mental link they shared, Primus hid himself (and Cybertron) away in a perpetual state of slumber to prevent Unicron from ever knowing his location. Primus can be rudely awakened, but only if the Seal of Primus, located deep within Cybertron, is broken. One such way is with an elaborate process called The Unbinding.
Fiction
Dreamwave comics continuity
Having captured Grimlock, Jetfire, Hot Spot, and Blitzwing, the Fallen began draining the energy from their spark cores, then directed that energy toward the Seal. Eventually, Grimlock managed to break free, and freed the others. Staying behind, Jetfire contacted Autobase, instructing Prowl and Bumblebee to erect a force field around his position, hoping to shield whatever was going to be released from the Seal. As the Fallen prepared to kill Jetfire, Primus awoke and, upon seeing the situation, stretched out his hand and "poked" the Fallen, condemning the wayward Transformer to a firery death. The Seal then closed, and the Autobots and the Decepticons agreed to seal the Well of All Sparks. Conflagration
