Mission 20

The Overlord will spawn when the Gate is destroyed. However, it is probably best if you destroy the vats and kill the fleas inside so they aren't accidentally destroyed during your fight with the Overlord. This will also give time for your Blue Force Generator to charge for your fight.

 

At the top of the level there is a secret.

 

The top left hand area, near the first secret, has a false wall you can walk through. In it you will find a computer which provides a hint for the final upgrade and can be hacked for a lot of research points. The bluetonium is also in this room which can help you against the Overlord.