The Krenim Temporal Weapon Ship was built in 2174, at height of the Krenim Imperium. It was created by temporal military scientist Annorax. When Annorax used the weapon ship for his first mission, to erase the Imperium's enemies from history, starting with their greatest enemy, the Rilnar. However, Annorax failed to fully understand the complexities of temporal mechanics. With the Rilnar gone, a vital antibody in the Krenim physiology against a certain disease was erased and 50 million Krenim died of a plague within a year. When Annorax used it a second time, his temporal miscalculation caused the Krenim Imperium to collapse, the colony on Kyana Prime to be erased, and Annorax's wife to be erased. Annorax spent the next 200 years trying to restore the Imperium to power and Kyana prime to exist by creating and erasing hundreds of species from history.
When the USS Voyager first encountered the temporal ship while crossing Krenim space, Annorax tried to erase them from history, but Voyager escaped. Voyager returned, along with the Mawasi and the Nihydron, and fought the Weapon ship. Voyager then performed a suicide attack on the weapon ship. With the sacrifice of Voyager the temporal ship itself was erased from history and the timeline restored. (VOY episodes: "Year of Hell", "Part II"; ST reference: Star Trek Encyclopedia)
However, despite this, Annorax still worked on the designs of the ship, even as the Imperium fell into disorganization. When the Vaadwaur Supremacy invaded Krenim space, the plans for the temporal weapon ship were secreted away in one of the Krenim colonies on Kyana Prime, protected from the Vaadwaur and the Iconian Heralds. When this colony was accidentally discovered by an Alpha Quadrant Alliance member aided by Captain Nog, the colony convinced them into using the ship's designs to recreate the timeship to change the tide of the Iconian War. The construction of the ship was succesful, receiving the name of its original designer, the KIS Annorax. (STO mission: "Time In A Bottle")
The Krenim Temporal Weapon Ship was equipped with a powerful temporal weapon. This beam weapon was able to push species, colonies and starships out of the spacetime continuum, effectively erasing them from history. However the weapon required precise, careful, and meticulous calculations to exact changes in the timeline, creating causality paradoxes.
The ship itself was powered by a temporal core. When active, the temporal core kept the ship, its contents and crew outside the normal space-time. This rendered the ship immune to all conventional weapons and protected the crew from the flow of time, effectively making them immortal. However due to the vessel's mass, it could not go faster than warp 6.(VOY episodes: "Year of Hell", "Part II")