The Trapped in Limbo DLC is included in the Atomic Pass. Requires the base game Atomic Heart to run.

Continue the story and learn P-3’s fate by diving back into the mad world of Limbo! Get through a combat extravaganza and Limbo-flavored challenges to learn P-3’s fate. Experience the contrast of worlds with unconventional mechanics selected to blow you away.

The Trapped in Limbo DLC features:

• Continuation of the story in new sections of the twisted Limbo world, meticulously created by the brilliant mind of Artem Galeev, Art Director of the Mundfish studio. Help P-3 to break free and, this time, explore what happens after the different ending of the main campaign.
• Insane gameplay with new rules packed into the Limbo-flavored favorite challenges. Run, jump, slide across platforms and fight bizarre enemies in a combat extravaganza.
• Marvelously-looking weapons and skills to earn as you collect apples and coins, progressing through the depth of P-3’s mind.
• Up to 7 unique weapon skins for P-3 to unlock, which you can equip in the Atomic Heart’s main campaign.

To activate the game, you need a Steam Account

• Login or register at the official
  Steam Website and download Steam client
• Install and start application, login with your Account name and Password
• Click the Games Menu.
• Choose Activate a Product on Steam...
• Follow the onscreen instructions to complete the process
• After the successful code verification, go to the "MY GAMES" tab and start downloading.