SAN FRANCISCO - Bethesda Softworks is building on the rock star debut of "Fallout: New Vegas" with the December release of a new chapter of the epic videogame exclusively for play on Xbox 360 consoles.

"Dead Money" software that builds on post-apocalyptic adventures in "New Vegas" will be available for download at console-maker Microsoft’s online Xbox Live service beginning December 21.

"The release of ‘Dead Money’ illustrates our commitment to creating entertaining add-on content for players to enjoy in already massive games," said Bethesda vice president of marketing Pete Hines.

Videogame studios are increasingly selling "downloadable content" that augments game play in titles bought on packaged compact disk-.

"New Vegas" predecessor "Fallout 3" was crowned Game of the Year after its release in 2008 and proved so popular that Bethesda expanded the action title with adventures in an array of downloadable software.

A "Broken Steel" addition to "Fallout 3" even modified the end of the original game to resurrect the hero, who had sacrificed himself for the sake of other survivors in the post nuclear war scenario.

"New Vegas" sold more than five million copies in the three weeks following its release on October 19, taking in "well over 300 million dollars," according to Bethesda.

The videogame was ranked by industry tracker NPD Group as the best-selling title for consoles and for personal computers in October.

"We recognize that there are many high-quality games that come out during this time of year," said Bethesda president Vlatko Andonov.

"We couldn’t be more pleased at how well the game has been received.

"New Vegas" is the fourth title in a winning franchise launched in 1997.