Terrance Clegg was a FBI agent-turned-human organ dealer for monsters.
At some point, he learned about the existence of monsters and all sorts of hunter's jobs in hunting them. Realizing around tens to hundreds of thousands of monsters lived in the United States, he later saw the potential for "additional revenue" through his "side job" as a human organ dealer. He claimed to Sam that, by doing this, he can keep monsters at bay and not attacking humans, since their need of food supply for human flesh will be supplied. With the aid of The Harvester and Marlon, he hunted many people that they felt would never be missed, like runaways, drifters, etc.
Later, he established a dark web site using TOR technology. With the website, he opened an auction where monsters could bid for human flesh.
Besides that, due to his involvement in both the FBI and monster community, he also learned about the existence and actions of the Winchester brothers, whom he called "legends".
After Wendy Hanscum went missing, he showed up in Oshkosh, Nebraska. Initially, he shared his findings of the so-called "Butterfly" serial abductor that he had pursued for 12 years to Sam, Dean, Donna Hanscum, and Doug Stover.
Somehow, he planted false evidence on Pastor Don Hankey to incriminate him for the girl's disappearance. In the interrogation, he unearthed the pastor's dark past to strengthen his accusation. However, Hankey's easy going manner and his repeated demands for a lawyer got under Clegg's skin. He tried to coerce Hankey, but Sam stopped him. Donna later interrogated the pastor, and he convinced her about his innocence. Sam later suggested resuming the investigation where it started, at Manny's Truck Stop Cafe.
Meanwhile, in Manny's Truck Stop Cafe, Dean learned strange facts from a Trucker and a Street Glass Cleaner. These pointed to Marlon as a possible suspect, he was visited by hunter Dean Winchester and Officer Doug Stover seeking information. Pretending to know nothing more than anyone else using Clegg's website aside from the fact that he turned people over to Clegg, Marlon showed Dean and Doug Clegg's website after Dean got violent. The hunters ask Clegg to trace the site after they realize they can't do it and he gives them the location of where the website is being broadcast from.
However, this turned out to be just a trap. The hunters and Terrence moved around the compound in groups of two (Dean and Donna, Sam and Terrance) searching for the butcher as Doug stayed back to watch Marlon. Marlon, then, revealed he was a vampire, quickly broke his handcuffs, and turned against Doug, overpowering him and turning him into a vampire. But he was later neutralized by Donna. When the two were alone, Terrance knocked out Sam and brought him to The Harvester' lair. Here, he revealed his true nature to Sam and told him he is aware of who he is as well as his reason for doing all his business with monsters.
Terrance proceeded with his business by using Sam for the auction, as he knew the monsters would revel in the chance to finish the legendary hunter. Bids for Sam were very high, they soon ended with an offer of five hundred thousand for his heart.
Terrance taunted Sam for hoping Dean would come and prepared to kill him with a gun. Fortunately, before all went too late, Dean showed up and killed Terrance with a fatal shot to his heart. With Donna killing The Harvester at the same time, Terrance's illicit operation ended.
- Terrance Clegg is the second FBI agent to be killed by Dean in the series following the ghost of Victor Henriksen and the sixth law enforcement officer killed by Dean. The other four are an unnamed cop, an unnamed Jefferson Starship, Sheriff Pat, and Deputy Donelly.
- Clegg's human organ harvesting operation began sometime in 2005 as Clegg mentions he has been pursuing the so-called "Butterfly" serial abductor for 12 years and the episode takes place sometime in late 2017. This is the same year Supernatural first aired.
- He resembles a DC Comics character named Professor Pyg.