Thanatos is a unique class of Tyrant that appears in Resident Evil Outbreak. It was created by renegade Umbrella Corporation researcher Greg Mueller and released during the Raccoon City Incident in 1998.
During the outbreak, Greg formed a partnership with Peter Jenkins at Raccoon University with the offer of helping him create a vaccine for the T-Virus. T-Blood, blood from a creature infected with the t-Virus, was offered by Greg to be obtained to create the vaccine. Greg had it in mind to get the blood from Thanatos. However, Peter discovered that Greg was helping him create the vaccine purely due to a personal vendetta, and that Greg actually sought to use the cure as leverage against Umbrella. As such, Peter refused to continue researching with him, and thus T-Blood from Thanatos was rendered useless. Thanatos, however, would not remain dormant.
An Umbrella Biohazard Countermeasure Service team was sent by Umbrella as part of Operation: Emperor's Mushroom. The objective was to enter the grounds of the Raccoon University and retrieve the Tyrant, or if the creature was too strong to contain, kill it and retrieve the remains. The operation was ready to go ahead from 0535 to 0635 on October 1, 1998, the very last moments available for the company.
Already aware of Umbrella's intent, Greg released his creation with the intention of wiping out the team. Some time around the time of the mission's initiation, the UBCS team, comprised of six operatives including Nicholai Zinoviev, made contact with the creature in the Back Square. Keeping their distance, the team continued to open sub-machine gun fire in an attempt to take it down. The tactic failed to take it down, and the other component of the plan - Nicholai collecting the desired T-Blood from behind - led it on a homicidal rampage, killing the ground-side components of the team, either unable or simply unaware of Nicholai for it to kill. Thanatos then carried off the body of one of the mercenaries to Greg for his researching purposes. Nicholai would begin preparations for another encounter, should it take place.
Thanatos then proceeded to pursue a rag-tag group of refugees throughout the university, possibly under Greg's orders. Whatever the reason, it failed, and the survivors managed to escape it's grasp. In fact, the survivors temporarily managed to knock Thanatos out with an electric shock, taking the pod with T-Blood with them. After Greg's assassination by Nicholai, and the activation of the C-4 charges he spent the time in hiding setting up, Thanatos again tried, and failed, to catch the survivors while they fled. While they escaped to the temporary safety of the Back Square, the C-4 went off, destroying a section of the university building and bringing the ceiling down around Thanatos. Though this severely injured it and cost it an arm, it also caused it to mutate into a monstrosity known as "Thanatos R."
After its mutation, Thanatos resumed its chase for the survivors outside the university. As they made their way to evacuation, Thanatos cut them off. Though it could have easily overpowered the group, their resilience proved an advantage. Still, Thanatos quickly resumed its chase at full strength. At the Front Square, Thanatos intercepted the survivors for the third and final time. They utilized an Ampule Shooter dropped by a dead soldier. By inserting a sample of the Daylight antigen - a vaccine to the T-Virus made with Thanatos' very blood - into it and shooting it into Thanatos, the survivors defeated it and escaped the city. However, Thanatos' body had the means to combat Daylight.
Thanatos was easily recognized by its sublime display of agility, brute force, and keen survival instincts. While chasing the survivors throughout the University, Thanatos displayed its quickness, easily outrunning all the survivors. Thanatos could quickly kill its victims by simply stabbing its claw into them. It was completely impervious to all small-arms fire, and the only known way to cause it harm was an intense dose of electricity. All this culminated into forging Thanatos into a perfect killing machine.
After it was weakened by the University's collapse, it went through a mutation that increased its size and strength. Upon becoming Thanatos R, it became characterized by its towering height; over 12 feet tall, a bigger and beefed up body, a single massive claw, and the loss of one arm.