After Mankind’s potential for good, The Son of the Devil, in defiance of his father, arrives in Los Angeles to send the supernatural creatures back to hell before they take over the city and find the hole in the Gateway before they take over the world.

What's LA got to do with it? If you're going to be specific about the location I feel like I want to understand why.

You've got two goals - send them back to hell before they take over the city, find the hell portal and seal it before they take over the world. They are very similar (in a visual sense) so pick one and go with it.

I think dpg is saying that "supernatural creatures" could be more explicit. If you mean demons, say demons. If you mean vampires, ghouls, goblins, etc say that! The reader needs to be able to see it in their mind. Just cos you say demons in the logline doesn't mean there can't be other creatures in the screenplay BUT keep the logline simple and visual.

Richiev covered the Inciting Incident so I won't mention that but I agree with him on all his points.

The problem with the new version is the word 'After' because if you start a logline with the word after it implies an?inciting incident, not?a motivation.
For instance, if you start the logline with: "After a train wreck..." You are saying there was a trainwreck and the story takes place after this major event.

But your logline reads funny because you say. "After mankind's potential for good..." This is confusing and causes the reader to have to decipher the logline.

You should rearrange the order a little bit:
"After Los Angeles is invaded by demons, the devil's estranged son, who believes in mankind's?potential for good, must..." (Then tell us what he must do)

This new one is a little more wordy, but I'm hoping to cut it down as I figure out what I want the logline to truly be. I hope this is better than the original.