From the publisher's website:

A new play is coming to Miskatonic University, and anyone who's anyone will be there for the opening performance. However, there's something sinister about this play, and the people who see it come back a little...different. As if the investigators didn't have enough to worry about before, now they must deal with the mind-warping effects of the King in Yellow, a play that tells a tale of madness and horror, opening a door for the Tattered King to enter through. This expansion introduces a major new mechanic to Arkham Horror - Heralds, who prepare the way for the Ancient One to arrive. In addition, old familiar faces will be turned against the investigators in ways they never expected. Featuring over 160 new cards, the King in Yellow heralds a darker age for Arkham Horror fans.