The Dead Matrix is carried within the chest of Nemesis Prime. Originally one of many Autobot Matrices of Leadership from a mostly undocumented corner of the multiverse, it was corrupted over time inside of a defeated Optimus Prime. As the body lingered, eventually very little of Primus's essence remained inside. Using Nemesis Prime, Unicron's tortured clone of the fallen Prime, Unicron was able to insert a portion of his own dark essence into the artifact. This newly created weapon, mirroring the relationship of the good Matrix with Unicron, would kill Primus should it ever be opened in his presence, which would be very bad indeed.