Primus is a character introduced in the Beast Wars television series.
In addition to its occasional invocation in prayer or in righteous anger by the Beast Warriors, Primus' name was also lent to a set of prophetical datatrax, known as the Covenant of Primus. From this document, the Megatron of this era took his name (as did, presumably, the original Megatron), and the scriptures foretold the events which completed the Beast Wars.
After the Beast Wars, Megatron intended to incorporate Primus' essence, the Allspark, into his own being and become godlike by stealing every Spark of every Transformer on Cybertron. Ultimately, this was merely one step of the Oracle's plan to reformat Cybertron into a technorganic paradise. After Megatron and Optimus Primal were plunged into the planet's core, the planet was reborn and the abducted Sparks were returned to new technorganic bodies.