Microsoft has once again secured the rights to exclusive DLC, this time in the form of two downloadable add-on episodes for Tomb Raider: Underworld.

The two episodes, called Tomb Raider: Underworld - Beneath the Ashes and Tomb Raider: Underworld - Lara's Shadow, will give the player "up to six hours of new gameplay and content" between them, and will be downloadable from Xbox Live.

Beneath the Ashes will be the first of the two episodes to be released, and will pop on on XBLA around Christmas time. Eidos has said the storyline follows events that take place after the Underworld story finishes, promising "an incredible new environment to explore, additional secrets to unlock and different enemies to fight".

Lara's Shadow, on the other hand "will introduce players to a new kind of playable character and create a unique Tomb Raider experience". It will be available to buy and download in early 2009.

Both episodes are so far unpriced, and are also both described as "single-player gameplay experiences."

"We're thrilled that Xbox owners will be able to extend their Lara Croft adventures with exclusive Tomb Raider: Underworld episodes," said George Peckham, general manager of Microsoft's global third party publishing.

"We look forward to continue working closely with our great publishing partners to deliver the most robust gaming experiences, only on Xbox 360."

Also rumoured to be an Xbox exclusive is a demo of the game, apparently due to be released on to Xbox Live Marketplace this month.

As for the full game, you'll be able to buy Tomb Raider: Underworld for PS3. Xbox 360, Wii, PS2, Nintendo DS and PC from 21st November.