Complex sentence: Lambert and Engineer Parker intervene, decapitating Ash, who turns out to be an android programmed by their employers to be set on board, so it could convince the crew to land and capture the creature in order to reverse engineer it.
Split into simpler sentences: Lambert and Engineer Parker intervene, decapitating Ash, who is revealed to be an android. The group learn that Ash had been assigned to the "Nostromo" to convince the crew to land and capture the creature in order to reverse engineer it.