A Stargate destroyer is a device built by the Ancients that can destroy a Stargate from another planet. It works by transmitting large amounts of energy through one Stargate to another, causing the gate on the other side to overload and explode from absorbing more energy than it can contain.
The Stargate destroyer functions by projecting a high-intensity energy beam into the event horizon of an active, outgoing Stargate. The energy is transmitted through the wormhole to the other side. Rather than reintegrating on the receiving end, however, the energy is absorbed into the gate. As time passes, energy continues to build up in the naquadah of the receiving gate. Because of the large amounts of energy being fed into the gate, it is able to bypass the 38-minute limit that disconnects ordinary wormholes. Eventually, the energy buildup exceeds the capacity of the naquadah in the gate, causing it to explode with the force of a two to three thousand megaton bomb, enough to cause a disastrous effect on the environment that would wipe out all life on a planet the size of Earth.
The device consists of a large, two-pronged emitter positioned in front of the Stargate, from which the energy beam is generated. A tube or conduit extends from the emitter into the ground, indicating that part of the device is buried (as indicated by the collapse of the area around the emitter followed by a large underground explosion seconds after the emitter itself was destroyed). The emitter is surrounded by a ring of smaller obelisks connected by a separate energy beam, which can be seen to spin at a steady rate. The obelisks generate a forcefield preventing entry through the open areas; however, a single unshielded doorway, being two halves of a small obelisk, allows access to the device. The forcefield does not completely cover the device, only blocking ground access.
It is not made clear why the Ancients would intentionally design such a device, though Anubis may simply have repurposed it to suit his needs. But there is the possibility that the stargate destroyer was originally a device used by the Ancients to recharge the energy source of DHDs or simply be an energy transfer device between planets or even as a weapon to use against Ori galactic incursions or as the power source the Ancients were going to use to dial Destiny. It may also have been constructed to bypass the 38 minute wormhole limit, but abandoned because of the consequence of extended use, similar to the Attero Device.
Anubis managed to locate the device, and used it in an attempt to destroy the Tau'ri. Earth was powerless to stop it, and had no means of reaching their offworld allies for assistance. Rodney McKay suggested using an Electromagnetic Pulse Generator to disable the device, but this failed to stop it.
Inspired by Jonas Quinn, Major Samantha Carter came up with the idea to fly the gate offworld using the X-302 hyperspace fighter. Using its hyperspace generator, the gate was flown to a safe distance and detonated harmlessly in space. Meanwhile, Rya'c, piloting a stolen Death Glider, was able to destroy the device. It is unknown if any attempts were made by Anubis to rebuild it, but is unlikely. (SG1: "Redemption, Part 1", "Redemption, Part 2")