I absolutely love it when a developer dives into my favorite genre; this can be with a full retail game or with DLC, and it seems that the latter has become increasingly popular over the last handful of years. The Zombie Island of Dr. Ned brought Halloween to Borderlands, Undead Nightmare for Red Dead Redemption, Festival of Blood for inFamous, and Lost in Nightmares for Resident Evil 5 (Zing.).
Not much is known about the Sleeping Dogs expansion, other than it’s called Nightmare in Northpoint and will bring a lovely dose of horror to the game when it releases on October 30th. If you don’t own the game already, you really should give it a try — I hear it’s pretty good.