Scimitar

20 shortish levels split into three episodes: Rear Entry, Knee Deep in the Sludge and Into the Blood. "The Source" - That's what they call it. No one quite knows what it is, apart from the birthing grounds of the monstrous army that invaded your planet. Your team was sent to destroy it, but the way in was heavily guarded, and now only you survive. You'll have to find another way in...