The Theta refers to "The Theta" which appears in the game most often of the normal Thetas, this is in other words, the main plot Theta.
Ramirez and Crispy were on the other side of the blast door when a large explosion causes Ramirez to send Crispy over to check it out.Crispy is infected and when Ramirez goes over to save him he is thrown against the blast door and he instantly died.
Right after fending off the large group of mutant and blasting the wall to the other side. Where the elevator awaits the monster appear and kill off many of the Delta Hazmat squad member and leaving Ethan Cole and Peter Chew
The Theta is finally confronted at the mothership entrance and finally killed by Ethan Cole after a hard battle.