Serious Sam HD: The Second Encounter will receive the Legend of the Beast DLC pack on May 15, extending the campaign and multiplayer experiences. The Legend of the Beast costs $4.99 and adds three new campaign missions, along with three new maps each for Brutal Survivor mode and Chaotic Versus mode. And again (weirdly), this is DLC for Serious Sam's second HD game, not the most recent Serious Sam 3.
In addition to the DLC, all Serious Sam HD: The Second Encounter's multiplayer modes – which include both co-operative and competitive-based modes – will be free for all through Steam on May 15. All you have to do is download the free, standalone multiplayer on or after May 15 and you're good to go.