Villengard was a planet located at the centre of the universe. Its weapons factories were known as the "Nightmare of the Seven Galaxies". (TV: Twice Upon a Time) Sonic blasters were produced there in the 51st century. (TV: The Doctor Dances) It had a moon in its orbit, though by the time the First and Twelfth Doctors alongside Archibald Hamish Lethbridge-Stewart and the glass avatar of Bill Potts visited, it had been partially destroyed. (TV: Twice Upon a Time)
In the last months of the Last Great Time War, the War Doctor enlisted the help of Dorium Maldovar to help him destroy the thirteen weapons factories of Villengard before the Dalek Fleet arrived to take control of them. Using a molecular fruit bomb, the Doctor transformed the factories into palm trees, creating a banana grove. (COMIC: The Whole Thing's Bananas) The Ninth Doctor used this event as a justification for why Jack Harkness should carry a banana instead of a sonic blaster. (TV: The Doctor Dances)
When the Eleventh Doctor set a test to verify Amy's identity so that she could unlock his message a thousand years later, one of the possible questions was 'Where are sonic blasters made?'. The correct answer was Villengard. (GAME: TARDIS)
Eventually, the "good" Dalek Rusty came to the ruins of Villengard's weapons factories, where he set himself up inside a tower to fight off other Daleks seeking to destroy him. The ruins became infested with Kaled mutants without their casings. The Twelfth Doctor visited Rusty to access the Pathweb and learn the identity of Testimony's glass avatar. (TV: Twice Upon a Time)
Behind the scenes
Villengard serves as a notable bookend in the history of Doctor Who, as it was initially mentioned in the first (non-satirical) story written by Steven Moffat (TV: The Empty Child/The Doctor Dances) and then appeared in the final story written by him (TV: Twice Upon a Time).