Mass Effect 2
Corporal Toombs was one of the Alliance marines at Akuze. He was a survivor of the thresher maw tragedy there; Cerberus was responsible for setting the thresher maws on his unit as part of an experiment. When he was found after the attack, Cerberus was delighted that he had survived and hauled him off for further study. Toombs was subjected to horrific ordeals including being injected with thresher maw acid.
Mass Effect Edit
After years of being experimented on, somehow Toombs managed to escape his confinement. Deeply traumatized by his experiences, he began systematically hunting down the scientists responsible and even enlisted mercenary help to do so. If Commander Shepard investigates dead Alliance scientists, they cross paths with Toombs on Ontarom. The situation takes a different angle if Shepard has the Sole Survivor background.
Toombs is about to kill the last scientist, Dr. Wayne, when the Commander reaches their location. Shepard can listen to Toombs' story and either talk him down (by promising to put the scientist on public trial for his crimes and allowing Toombs to get psychological treatment) or allow Toombs to kill the scientist, believing he has earned his revenge. Shepard can also kill the scientist personally, using Spectre authority.
Alternately, Shepard may shoot Toombs to prevent his act of vengeance, allowing the scientist to live. The Commander then dispatches the Fifth Fleet to pick the doctor up and put him on trial. Depending on circumstances, if Shepard is a Sole Survivor, Toombs will point out that the vids say Shepard was the only one to escape Akuze, adding "who am I to argue?" before killing himself.
Mass Effect 2 Edit
If Toombs survived his encounter with Shepard in 2183, he will send the Commander a message at some point in 2185, entitled What the hell?. He has discovered Shepard's current connection with Cerberus and, feeling betrayed, promises dire retribution should they ever meet again.
Shepard may also overhear Galactic News reports describing Toombs' other activities. If both he and Dr. Wayne survived, an independent inquiry based on their testimonies is underway against Cerberus. If Wayne died, the inquiry dismisses Toombs' claims as he is seen as disturbed and because of insufficient evidence to validate his story.