The chrono deflector was a device built to manipulate the spacetime continuum. Although it was capable of allowing someone to travel through time, it produced a great deal of tachyon radiation - making it lethal to anyone who hadn't been inoculated with chronexaline. Further, the tachyon radiation levels meant that the device would burn itself out after one trip.
In an alternate timeline, Korath had invented this device somewhere around 2404. Admiral Kathryn Janeway attempted to gain this device to correct what she felt was her greatest error - not taking Voyager into the Borg transwarp network in 2378. She arranged to get Korath onto the Klingon High Council as payment for the deflector, but had to resort to stealing the device when he demanded her shuttle's ablative armor hull technology in addition. Janeway installed the deflector on her shuttlecraft, with the help of Harry Kim, and was successful in travelling back to 2378. (VOY: "Endgame").