Joshua's trumpet acts like a sonic cannon and the sound waves it generates can turn anyone or anything it's aimed at to dust with one blast. Larger objects are only partially disintegrated.
Joshua's Trumpet was first discovered in 1893 by Warehouse 12 after people started vanishing into thin air leaving only a strange dust behind. At least two Warehouse agents were killed in trying to obtain it and relocate it to Warehouse 12. Whether it physically was delivered to the Warehouse is unknown, as it was later used by Helena G. Wells and Sir James Eddington as the propulsion source for a rocket Helena was building.
Helena's exact intentions for the rocket are also unknown as it was deliberately launched by Vincent Crowley in an attempt to prevent the Warehouse being relocated to America. Due to Helena's interference, the rocket was not launched on its intended trajectory towards Germany but into the atmosphere, killing Crowley. The Horn, as part of the rocket, remained in orbit for 69 years until the rocket re-entered the atmosphere in 1962 and crashed near a farm in rural Ohio.
The Horn became an item of interest to Warehouse 13 shortly after local birdwatcher, Doug Varley, disappeared, and two teenagers reported seeing a UFO "ray gun" a cow after hearing a strange noise. Agents Jack Secord and Rebecca St. Clair attempted to retrieve the artifact, but were unable to do so as the Horn was still attached to the propulsion drive system, causing it to continually fire bursts of concentrated sound waves. It was believed that the agents destroyed the Horn by shooting a propane tank next to the remains of the rocket.
However, the Horn was not destroyed, although the remains of Helena's rocket were. Instead, it was recovered by the young Daniel Varley, who believed his father - the bird watcher who had disappeared - had been abducted by aliens. After moving to Pennsylvania and despite spending large periods of time in various mental institutions, Daniel became an accomplished astronomer and obtained a job with SETI which allowed him to devise a way to activate the horn, using it as a means of contacting a small planet, GL-581. As Daniel was unaware of the Horn's abilities and didn't attempt to use it, combined with Jack and Rebecca's report that the rocket was vaporized, the Warehouse was unaware of the Horn's continued existence.
Daniel was able to use the horn for the first time in 2011, working out of the abandoned SETI base in Pittsburgh. Initial attempts to use it to reach GL-581 resulted in the disappearance of a painter and a utility worker among others, as well as destroying part of the rotor assembly on a helicopter.
Pete and Myka were able to deduce that Daniel had the Horn, and were able to track it to the SETI base. Daniel's attempts to use the Horn to contact GL-581 were hampered by a lack of power to project the sound waves into space, leading to the deaths of four people in his previous attempts. He was later convinced by the projection of Helena to release the Horn into Pete and Myka's custody. The Horn is currently stored in the Warehouse.
Using the artifact
Joshua's Trumpet is activated by blowing through the mouthpiece. Both Helena G. Wells and Daniel Varley were able to operate it mechanically by connecting an external compressed air supply to the Horn, while Daniel was able to project the sound further by attaching the Horn inside a satellite dish at the SETI compound.
Real World Connection
According to the Abrahamic Religions, the Israelites walked around Jericho while carrying the Ark of the Covenant for seven days, on the last of which the priests blew trumpets of rams' horns and shouted, causing the walls of Jericho to suddenly collapse. Joshua 6:20 from the Old Testament book of Joshua refers directly to this:
20 So the people shouted, and [the priests] blew with the horns. And it came to pass, when the people heard the sound of the horn, that the people shouted with a great shout, and the wall fell down flat, so that the people went up into the city, every man straight before him, and they took the city.