You missed 泊りで in the first clause of the first sentence.
The core of the main clause is 旅館で何が起こるのか(i.e., that's an interrogative sentence).
何か is omitted at the end of the second sentence (thus, this is also an interrogative sentence).
As for the third paragraph, 描き下ろし means that the artist newly drew the illustration for the cover jacket, not just "drown".