The alien is you, and you are not welcome.

The Keeper [Black Mesa: Blue Shift OST]

Joel Nielsen - BMS - Apprehension

I did not make this it was made by Joel Nielsen on the black mesa team. All credit goes to them. 
-Black Mesa SoundTrack 05 Alien

did not make this it was made by Joel Nielsen on the black mesa team. All credit goes to them. 
-Black Mesa SoundTrack 08 Mind Games

did not make this it was made by Joel Nielsen on the black mesa team. All credit goes to them. 
-Black Mesa Xen SoundTrack 10 Harbinger

I did not make this it was made by Joel Nielsen on the black mesa team. All credit goes to them. 
-Black Mesa SoundTrack 13 Respite