The House System
The Darkest House is compatible with the game you’re already playing—in fact, it’s made to be integrated into your current campaign. Within the Darkest House, you’ll use the House System. You’ll quickly and easily convert your existing characters (it takes about five minutes) and use those stats while in the Darkest House. Once your characters emerge your players will return to their regular setting using their regular character sheets. The House System is a sort of RPG Rosetta Stone, allowing easy translation from any game system—and even letting you bring characters from different games together.
You can launch a campaign centered around the Darkest House. But the Darkest House really shines when you introduce it to existing characters from your ongoing campaign.