John Carpenter's In The Mouth of Madness. A detective working for an insurance company investigates the disappearance of a horror novelist (a la Stephen King, Dean Koontz and Clive Barker rolled into one).
(The guy who wrote the screenplay was an exec at New Line. That company used to have an expense account back in the 90s until he and Adam Sandler blew 6 figures in Vegas playing blackjack. Started a new company policy: No More Company Expense Accounts. lmao)