Omega Arsenal

The Omega Arsenal was a collection of forbidden weapons locked in the Time Vaults on Gallifrey, (TV: The Day of the Doctor) beneath the Capitol near the planet's core. (PROSE: The Day of the Doctor, A Brief History of Time Lords)

Rassilon locked the Moment away in the Omega Arsenal after he deployed it against the Nestenes. (PROSE: Pandoric's Box) It was kept in the deepest time vault, Time Vault Zero, for billions of years. (PROSE: The Day of the Doctor)

Huxley speculated that weapons used by the Great Houses during the War in Heaven — such as the "Greater Key" — would in the Peace be taken back to "some impregnable arsenal". (PROSE: A Farewell to Arms)

The Tear of Isha, which was used to collapse black holes was kept in the Arsenal, (PROSE: Engines of War) as was a weapon capable of generating a Null Zone, (AUDIO: The Heart of the Battle) the Orphaned Hour, (COMIC: Strange Loops) and the anima device. (AUDIO: A Thing of Guile) Sentient life forms were also kept in the Arsenal, including a Dreadshade. (AUDIO: Dreadshade)

Over the course of the Last Great Time War, the Time Lords used all the weapons in the Arsenal against the Daleks save the Moment. (TV: The Day of the Doctor) The Twelve was placed in stasis in the Arsenal after the Uzmal incident, however she managed to orchestrate an escape by manipulating the Dreadshade and also freed all the sentients, though they were later recaptured and returned to the Arsenal. (AUDIO: Dreadshade) On the last day of the War, the War Doctor tricked the War Council into opening Time Vault Zero, so he could claim the Moment. (TV: The Day of the Doctor; PROSE: The Day of the Doctor)

Arriving in the Capitol after escaping the Cyberman invasion of Karn, the Twelfth Doctor called for the Arsenal to be opened, only to be met by Rassilon, allied with the Cybermen. (COMIC: Supremacy of the Cybermen)

Behind the scenes
The "Fourth Dimension" feature on the BBC's Doctor Who website suggests that the Omega Arsenal is a reference to Omega.