The temporal core is a device at the heart of the Krenim temporal weapon ship, which allows the vessel to remain shifted out of phase with normal space-time. This protects the ship against changes in the timeline caused by temporal incursions, and renders it impervious to conventional weaponry. Shutting down the core activates every alarm on the ship.
In 2374 of an alternate timeline, Obrist shut down the temporal core in the midst of a battle with the USS Voyager and her allies, making the vessel vulnerable to attack. Voyager subsequently made a suicide run on the ship's midsection, destabilizing the core and causing the weapon ship to erase itself from history. (VOY: "Year of Hell, Part II")