The Grigari are a powerful civilization from the fringes of the Beta Quadrant, in territory that may actually border the Delta Quadrant. Their culture is based on piracy and, unlike the Ferengi, they will resort to unsavory tactics to make profit.
They appear to be a type of mechanical based lifeform. Its been noted that no Grigari construct has been able to overcome a life or death situation without attempting to bargain for its life.
In 2267, the Enterprise encounters a Grigari vessel, allied with former WWIII leader Adrik Thorsen. They demand of Kirk and bridge crew full cooperation for the whereabouts and apprehension of both Zefram Cochrane and wife Nancy Hedford/Companion. Mr. Spock also notes that the Grigari commander has cybernetic growths that extend into the microscopic level for better access to systems. (ST novel: Federation)
In 2374, a group of Grigari were hired by Obanak Keelen to obtain the Orbs of the Pah-wraith. These mercenaries would have numerous encounters with the crew of Deep Space 9. This group of Grigari would be killed on Deep Space 9, either by the Starfleet crew or under the influence of the Pah-wraith possessed Obanak. (DS9 game: "The Fallen")
The Grigari feature heavily in the alternate timeline generated by the start of the War of the Prophets. They were encountered when the Red Wormhole was formed and a Dominion fleet was sent into the anomaly to determine whether it connected to the Gamma Quadrant. However, it was learnt that the wormhole actually connected to the Delta Quadrant. All the crew of the ships were believed to have been killed with Weyoun being the only survivor. He returned to the Alpha Quadrant, changed by the Pah-wraiths, with the Grigari who served as the military arm of the newly formed Bajoran Ascendancy.
The Grigari forces attacked the Dominion and Cardassian force to bring an end to the Dominion War. The offer of surrender was given but was refused. As a result of this, the Grigari wiped out all the worlds of both races leaving nothing standing. In 2375, the Grigari freed Thomas Riker from his imprisonment on Lazon II. The former Starfleet and Maquis member decided to join the Grigari forces.
Several Grigari forces were invited to Sector 001 in 2388 to take part in trade negotiations. However, the Grigari were confused by a transmission and opened fire. The Enterprise-F was destroyed by the Grigari in the process. They would be responsible for the destruction of Earth, spread a Nanospore plague to the majority of Humanity almost wiping out the species and destroy a large Klingon armada. They were later present on the planet where the Guardian of Forever was located to stop Starfleet's attempt at using the ancient machine to travel back in time to defeat the Ascendancy, as the battle closed the Grigari detonated a singularity bomb that wiped out everything on and around the planet. (DS9: novel series: Millennium)