Tachyons are quantum particles that travel in time. They are created as a side effect of time travel.
Rex Salazar and Bobo Haha went to investigate the Hall in Cairo, Egpyt. While exploring one of the chambers, they discovered tachyons, leading Holiday to believe that Van Kleiss had tried to create a time machine in reality it was Breach's enchanced abilities. It also seemed that some of the tachyons were far older than others.
Caesar Salazar was able to contain the Tachyon particles which Rex was then able to use to turn them back into Breach.