On the order of Rassilon to prevent the War Doctor from stopping their plan to seal up the Tantalus Eye to safeguard Gallifrey from the dematerialisation cannon the Daleks planned to use to destroy the Time Lords and erase them from history, allowing them to end the Last Great Time War, Karlax took command of a Battle TARDIS and four others by using a tracker placed on Cinder when she'd been on Gallifrey.

The Battle TARDISes managed to fire a time torpedo at the Doctor's TARDIS, suspending it in a time bubble temporarily before they were attacked by a squadron of Dalek stealth ships. The Battle TARDISes managed to destroy one of the stealth ships, but the Daleks fought back and destroyed the TARDISes.

Karlax's Battle TARDIS died in the battle; like the rest of the squadron, the Battle TARDIS he was on was torn apart, the interior dimensions were exposed to space, and when the Doctor rescued him, pieces of wall and console were all that was left except what was hanging in space. (PROSE: Engines of War)

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