A large-scale catatstrophe following the First (Giant) Impact and the Second Impact, as foreseen by human.
It had been said that this would be started by from alien Angels who came into contact with Adam stored underground beneath Neo-Tokyo-3, and therefore EVA and NERV were there to protect this from happening.
However, the truth is that they were there precisely to start the Third Impact, the real objective being the Human Complementation Plan.