My favorite "mini" expansion to Arkham Horror! The new "Heralds" that help the Ancient one awaken from his slumber and new gate tokens, which make gates take away stamina/sanity and even move around the board, also add to the depth and complexity of the game. This in my opinion, makes it even better!
Every Arkham expansion is a good development to the game, with new cards in all the decks you want them in. For that alone I recommend Lurker, but the new gate markers (that move, devour, etc.) and the herald himself add to the chaos that you might think you've finally mastered. Great job!