If anything, you could credit it to both H.P. Lovecraft as well as the creators of the game - they invented much of the story and most quotes/speeches that were given (with the exception of Pious Augustus's speech before condemning Roberto into the Tower of Flesh) were invented by the creators with inspiration from Lovecraft's mythology.

Now, now. While Eternal Darkness is in it's entirely Lovecraftian, you can't forget the gothic era of poetry's effect.

C'mon.. they friggin quote Poe before the game even starts.