Martian Gothic: Unification
Готика Марса: Кровавая сторона планеты - Russian spelling
You control a team of three: Kenzo, Karne and Matlock. They enter the base from different airlocks and try to discover what happened. The reason for the whole "Stay alone, stay alive" warning is that if your team members come together they end up forming into a disgusting mass of flesh -- picture the thing from Leviathan -- called a Trimorph.
As you proceed though the base you find clues as to what occurred. In addition, you run into Trimorphs, some of the non zombiefied former base crew, that are bent on your destruction. They come in two sizes -- the small spidery kind which hack at your shins and jump on your face or the huge "better run like hell" monstrosities that will tear you to pieces.