Three different clones of Yelgrun served as a Vorta overseer for the Dominion in 2374. One month into the Dominion War, Yelgrun was part of an operation to weaken Federation and Klingon forces following the destruction of the Torros III shipyards. After the Federation had been led to believe a facility for breeding Jem'Hadar in the Alpha Quadrant was on a planetoid in the Tong Beak Nebula, Yelgrun and his troops were charged with observing the activities of the USS Lambda Paz, which was sent to verify the authenticity of this intelligence finding. Once that ship's reconnaissance was completed, Yelgrun contacted Weyoun to report that the Federation had taken the bait and request reinforcements.
During the Siege of Sector 21607, the Jem'Hadar under Yelgrun's command learned that he had lied to them about the existence of breeding facility on the planetoid. In response to this slight, First Teron'tokal and his subordinate troops killed Yelgrun. (Star Trek: Lambda Paz: " Revenge a Dish Best Served Cold")
A subsequent clone of Yelgrun was sent to negotiate with a group of Ferengi for the safe return of Ishka, the paramour of Grand Nagus Zek, in exchange for Keevan, a Vorta who had been captured by Starfleet several months earlier. Ultimately, Yelgrun was tricked into exchanging Ishka for the animated corpse of Keevan. His two Jem'Hadar were killed, and he was captured and turned over to Starfleet. He presumably activated his termination implant, as all Vorta were required to do if captured, since yet another clone of him activated soon afterward. (DS9: "The Magnificent Ferengi", Star Trek: Lambda Paz: The Tides of War, Part 1)
This next clone of Yelgrun probably underwent minor genetic alterations, so that he would be a little more cautious with species known for deceiving business clients, such as the Ferengi, or who regularly practiced piracy, such as the Sindareen.
Yelgurn had been sent to meet with Sindareen representative Tor Vot, who had provided information to the Vorta that would allow the Dominion to attack and invade Betazed. He told Tor Vot that, once the Dominion won the war, the Sindareen would become valued members of the Dominion, even stating that any enemy of the Federation's was friend of the Dominion's. These statements may have been lies to assure future cooperation from the Sindareen.
Yelgrun served alongside Gul Enic Hadar, who was seeking to avenge his brother's death. The two had a very contentious relationship when they commanded a fleet of Dominion and Cardassian warships that lured the Lambda Paz to the edge of the Briar Patch with a fake distress call from a Vulcan freighter under attack by the Sindareen. Once that was accomplished, the Dominion fleet crippled the Lambda Paz and sent boarding parties. Yelgrun stayed behind on the Dominion flagship to patrol the Patch while their newest trophy was being repaired. He was very demeaning towards Hadar, reminding the Cardassian that he was only involved in the operation because of a "petty quest for vengeance." (Star Trek: Lambda Paz: " The Tides of War, Parts 1 and 2")