(covers information from several alternate timelines)
The Teral'n was a Romulan staff weapon that had two retracting blades.
In 2387, Nero kept a Teral'n on board his ship, the Narada. Nero had the weapon in hand when the Narada accidentally traveled back in time to 2233, creating the alternate reality. He used it to kill Richard Robau, captain of the USS Kelvin, when he informed him of the current stardate. (Star Trek)
Nero's acquiring of the Teral'n is explained in Star Trek: Countdown. In the book, the Teral'n (referred to as the "Debrune Teral'n") was an ancient weapon which served as the greatest symbol of the Romulan Star Empire. In the book, the praetor was in possession of the Teral'n when he and the ruling council of the Romulan Senate were beamed aboard the Narada following the destruction of Romulus. Angered that the Senate refused to believe Romulus was in danger, Nero took the Teral'n for himself and used it to kill the entire Senate.