Major Sergei Vallarin was a member of the first official Russian SG team under the command of Colonel Alexi Zukhov. He and his team were sent to the planet P2X-338 after SG-1 discovered it to be a location a secret Russian team had been sent to by Russian Army Intelligence.
The two groups arrived at the entrance to a ziggurat which they believed had been used by the Goa'uld masquerading as the Babylonian god Marduk. As soon as they entered the ziggurat, however, everything went wrong. Zukhov set off a booby trap when he found Marduk's sarcophagus which locked the entrance to the ziggurat, the entrance crushing Lt. Marchenko in the process. Soon after this Lt. Tolinev, another of Vallarin's team, was attacked by a strange creature which was housing Marduk's symbiote, poisoning her and sending her into shock.
Vallarin, disobeying the order to stay with Daniel, left his post when he heard a strange noise down a hallway they were next to. The creature chose this moment to attack him but instead of biting Vallarin as he had done to Tolinev, the Goa'uld symbiote Marduk instead chose to take Vallarin as his new host.
As such, Marduk took control of Vallarin's body, presumably suppressing Vallarin's original consciousness as well.
Meanwhile, Daniel studies the writings on the walls of the tomb to realize that there is a ring transporter in the temple, and Teal'c locates a secret panel that exposes Goa'uld control crystals, which would allow SG-1 to activate the ring transporter.
As they get ready to leave, Marduk shows up to threaten his revenge with Jack telling the Goa'uld that they always say that.
As the ring transporter dematerializes the four members of SG-1 and Tolinev, Major Carter activates a ten second timer on a large quantity of C-4, killing the host, Goa'uld, and collapsing the already very unstable tomb. If the Goa'uld survived the initial explosion, he would probably soon suffocate under the tons of stone and dust that would subsequently bury him. (SG1: "The Tomb")