The phage torpedo was a supposed form of torpedo developed by the Vidiians. Its namesake is the Vidiian disease known as the phage.
Reginald Barclay claimed the USS Voyager had phage torpedoes, along with Borg interquadratal warp drives and Hirogen hunting sensors, in order to trick the Ferengi into closing the geodesic fold in 2377. (VOY: "Inside Man")
It is not known whether this technology existed or whether it was made up by Barclay. If it did exist, its exact relation to the phage is unknown.