A weapon of immense power designed by the Ancient Merlin to destroy ascended beings -- specifically meant for the Ori. Also called the "Sangraal" in Earth mythology, is an energy transfer device that channels energy into the dimension occupied by ascended beings. In theory, the weapon destroys ascended beings by interference, as two standing waves cancelling out one another.
Believing the weapon to be linked to Merlin's amulet, SG-1 followed the clues left by Merlin and complete hazardous tasks set by Morgan Le Fay, finally meeting the dying Merlin himself and discovering the weapon in pieces.
Before he died, Merlin downloaded his consciousness into the mind of Daniel Jackson, using a device similar to the Ancient repository of knowledge. This enabled Daniel to use the Ancient assembler and partially complete the weapon. After he was captured by Adria it fell to Vala Mal Doran to complete the assembly of the weapon using instructions left by Daniel.
The weapon was eventually deployed by Daniel (who had been turned into a Prior but retained his sense of self, thanks to the protection of Merlin's consciousness). Daniel activated the device and sent it through the Supergate to the Ori home galaxy. Only months later was it confirmed that the weapon worked, and the ascended Ori were obliterated.
Arthur's Mantle - Once on the other side, Daniel reads the display on Merlin's computer, explaining the weapon he was building and the location to where he left it.
Camelot - Kvasir theorizes about how Merlin's weapon works, while SG-1 gains a vital new clue about where to find it.
The Quest, Part 1 - SG-1 discovers the planet where Merlin likely hid the Sangraal, but must get past a series of obstacles set by Morgan Le Fay before they can reach it.
The Quest, Part 2 - As the Ori forces close in, SG-1 race to complete the weapon.
The Shroud - The newly activated weapon is sent through the Ori's super-gate.
Stargate: The Ark of Truth - SG-1 learns from Adria that the anti-Ori weapon did indeed work, wiping out the evil ascended beings.