The Debrune teral'n was a trident shaped weapon with two retracting blades that was an important artifact within the Romulan Star Empire. This ancient weapon was the greatest symbol within the Romulan government and was always guarded by the Praetor, who had the traditional task of carrying the trident and responsibility of finding a home for it.
In 2387 when the Hobus star exploded leading to a powerful shockwave spreading throughout the quadrant which brought about the destruction of Romulus, the Imperial Romulan Senate managed to survive the destruction of the Romulan homeworld. They were rescued by the Romulan miner Nero who believed that the Star Empire was now dead. Thus, he killed the Praetor and several members of the Senate whereupon he took the Debrune teral'n for himself and hatched a plan to destroy all those who brought about the destruction of everything he cared about. (TNG comic: "Countdown, Number Three", TOS movie: Star Trek)