Chroniton torpedoes were weapons capable of penetrating an opponent's shields because the shields were in a state of temporal flux. These weapons were used by the Krenim Imperium and the Tandaran Empire of an alternate universe. (VOY episode: "Year of Hell", DTI novel: Watching the Clock)
In 2373, a Tandaran warship used a track-jumper to enter the primary universe and then assaulted the Department of Temporal Investigations branch office on Aldebaran III with chroniton torpedoes. (DTI novel: Watching the Clock)
In 2374 of an alternate timeline, a fragment from a Krenim torpedo lodged itself in the haul of the USS Voyager, leaking chroniton radiation into the ship. This radiation later caused Kes to jump through time, a process the Doctor was eventually able to stop. (VOY episode: "Before and After")
In the main timeline, a Krenim chroniton torpedo was revealed to be the cause of an incident when Voyager was briefly fractured across multiple time periods (VOY episode: "Shattered"), the same attack resulting in the abduction of an 'alternate' Janeway taken from a period just before Voyager was taken to the Alpha Quadrant, with this Janeway continuing to reside in the prime reality (VOY novel: A Pocket Full of Lies).