The Resonance Cascade woke Alma. Play Black Mesa haunted by F.E.A.R. 3, with Reflex slow-mo on a key.
Play the whole Black Mesa campaign as Gordon Freeman, except the Resonance Cascade woke something else up.
What F.E.A.R. 3 brings in (played straight from your own F.E.A.R. 3 install while you play, never copied):
Needs: Black Mesa and F.E.A.R. 3, both installed. Solo.
Notes: the mashup turns on Black Mesa's cheat setting so slow-mo works. A small companion program runs beside the game and closes with it.