Forum:Paracide Parasite on a full field?

I just wanna ask this quick and, hopefully simple question. A friend of mine used Paracide parasite, and it was placed in my deck. But when I drew it, I had 5 monsters on my field already.

So my question is: What exactly happens then? Do I have to destroy a monster to have him still summoned? Does he go to the graveyard? I'm stumped. My fried too, apparantly. We just decided to give me the 1000LP damage and move the Parasite to his graveyard.

But still, i really want to know what the legit procedure is.