There (is/are) in our deepest nightmares a silent monster waiting for its next victim.

Not sure what the question is but if your looking for clarification then here. People think dream on or dream big but in reality, things just sometimes don't work out. That's why there is fear in everything done. Our biggest fear is really not meaning anything to the world and that is our silent monster. If you let fear conquer you, then you've become the next victim. Always believe in a bigger greater reality and it comes into play. Einstein once said: " Reality is merely an illusion, albeit a very persistent one." Just learn to out beat your reality and you'll be fine. We as humans only have access to 10% of our minds. Imagine what we would be capable of with 100%. Just let that soak.

There IS ...

Explanation:

The verb (is) must agree with the subject (monster).

Don't fall for the trap in thinking that "nightmares' is the subject simply because is follows closely behind the verb.

Maybe this will make it clear ...

Recast the sentence to read:

In our deepest nightmares, there (is/are) a silent monster waiting for its next victim.