In all seriousness, Primus was killed by Orcus while he was Tenebrous, in the prologue of The Great Modron March. This information never plays a part in the adventure itself, so it was mostly window dressing. Huge, campaign shaking window dressing. Another modron ascended to take Primus' place, but I don't believe this happened during The Great Modron March because the epilogue has Tenebrous still in Primus' throne room.
The original Primus, now a distinct entity, was statted as a vestige in Dragon 341.