Eldritch horror descending from the sky

You find a book lying on the front of your doorstep. You open and find what looks like so sort of incantation. Thinking that this is some sort of prank, you the incantation aloud. All of a sudden, there are screams and the sky goes gray. You look up a see what looks to be a monster in the clouds. The world is doomed, and it’s all your fault.