The Idol of Thoth is one of the best Call of Cthulhu scenarios I've ever read, and possibly one of the best introductionary scenarios for both new Players and Keepers alike.

It's very well paced and provides everything a new Keeper needs to run the players through an interesting investigation into the theft if the titular object; NPC and locations are described well, but always leaving it open enough for the Keeper to add their own twists and spins to them, which I see as a plus in my book.

This is a very investigation heavy scenario, so if you want more action and even more horror, that's something you'd have to add yourself. But if you're interested in a solid investigation scenario to have as the start of your campaign, or fit it into an already running campaign, I can't recommend this scenario enough!