The Sangraal, also known as an anti-Ori weapon, anti-ascended weapon and in some cases the Holy Grail, is a device created by Merlin to destroy the Ori. After the Sangraal was conceived and in the process of being constructed, it became associated with the legends of the Holy Grail.
Though the specifics of how the device works is unknown, the Asgard have surmised its functions based on the datalog within Arthur's Mantle, discovered in Merlin's vault at Avalon. According to Kvasir the device draws and releases energy in a manner very similar to a Zero Point Module, but instead of releasing the energy into real space it is sent into the dimension occupied by Ascended beings. This energy acts as a form of interference, like positive and negative charges canceling each other out, that negates the powers of every ascended being within range, killing them. (SG1: "Camelot")
Because the Others feared Merlin's research into a device that could be used against them, Morgan Le Fay was sent to destroy the device and kill Merlin. She managed to destroy the Sangraal, but came to believe the Ori were a true threat so she preserved Merlin in a stasis pod and hid him on a planet whose Stargate could be reached by completing the quest for the Sangraal. The address of the planet the Sangraal was supposedly hidden on was composed of the addresses of three others: Castiana, Sahal, and Vagonbrei. The Sangraal's location and secrets were known by the Ancients, Dr. Daniel Jackson, and Anubis when they were both ascended but because of the Others interfering, they wiped the knowledge of the Sangraal from Daniel and Anubis. After being removed from stasis, Merlin used a special device to try and build another Sangraal, but his body had deteriorated too much while in stasis, so he downloaded his consciousness into Jackson's mind with Merlin's physical body later dying. Daniel later attempted to build another Sangraal, but was stopped by Adria, who captured Daniel and obtained the construction device. (SG1: "The Pegasus Project", "The Quest, Part 2")
She hoped to use Daniel to complete the Sangraal so that the Ori might destroy the Ancients without having to risk themselves, but Daniel, aided by Merlin's consciousness, used Adria to complete the Sangraal. They developed a plan to use it against the Ori. Daniel, transformed into a Prior, used the Supergate to send the Sangraal to the Alteran Home Galaxy. The fate of the Ori remained unknown until SG-1 traveled to the Alteran Home Galaxy and learned from Adria that the Ori, with the exception of herself, were indeed exterminated. (SG1: "The Shroud", "The Ark of Truth")