Ahhh De Profundis, what art thou? I mean, apart from 110 PDF pages of text with the occasional photo touched up to look like an old photograph.
Simply put it's a Story Telling game where people can use the framework it sets up to create a living story in a Cthulhu Mythos world. Although it does drift off-topic a little to suggest how you can use the same ideas to play in other worlds, such as love affairs in Spain during 1850, that is quite obviously not what they were aiming at. This game is soundly rooted in Lovecraftian horror.
The game presents no rules, no conflict resolution, no list of investigator skills and no list of weapons or monsters to help the GM, because there is no GM. When playing, everyone takes an equal role in helping to tell the story, a true collaborative story telling experience.
A word of warning before I go on. This book is written weirdly and you may find the going a bit heavy. It's written as a series of letters ( an epistolary approach ) . The letters are written in-character by a person that has found a creepy book called De Profundis. As the letters go on this mysterious book is revealed to contain a game about writing letters...which is what the game De Profundis is really about. So it's a bit of a circular-reference and odd.
...and that's all there is to the core of De proundis. You and your pals, write in-character letters to each other. With the preceding sentence I just presented the core of the game. You might now be thinking, "well if that's all it is why should I buy the game?" That's a good question. Yet I recommend that if the idea appeals to you, you should get your hands on a copy of the book (or PDF).
The confusing presentation of the game I mentioned, is actually a mood and an attitude that you need to adopt to get the most from the idea. Most of the text of this book presents the correct attitude and mood required for the game. It suggests how to best approach the letters, how to form your mind into the right receptacle to best appreciate the half-insane letters you'll be sending or receiving. I think it very unlikely that the game will be as good as it could be, if any player has not spent the time to read these rules and appreciate the attitude needed to play.
With the appropriate attitude and approach you will find yourself hand-writing letters on quality paper with a fountain pen, and paper-clipping photos (with a little Photshopping perhaps) of weird and inexplicable things to the letter before posting. Thus you see that the game will take time and effort and a love for the game. It's not a case of turning up on Saturday afternoon with pop n chips. This is a game where you're going to sit at a table in a darkened room, suck on a faux pipe wearing glasses you don't need, just to get in the mood to play your part and write a letter.
If you want to play this game, you'll be setting aside a couple of hours a month for the next few years and really exercising your mind as you get into character.
I give this game-book a big thumbs-up.