Thanatos is 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.
Like other Tyrant versions Thanatos had an exposed heart, though a metal case was fitted over it protecting the characteristic weak point.
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, beefed up body; a single massive five-fingered claw; and the loss of its right arm. To compensate, the remaining left arm became gigantic and elastic, able to stretch great distances and swung about like a flail.
In the paired bad ending for Kevin and Jim, Thanatos is shown to have survived the t-Virus vaccination. It is unknown exactly how the Tyrant survived, as no other Tyrant was exposed to a vaccination.
Birlew, Dan (2004). Resident Evil Outbreak Official Strategy Guide. Indianapolis: BradyGames.