Camouflage shield

A camouflage shield was a device Sontarans used. Per its namesake, it could disguise anything the user desired to blend into the environment. However, once the shield is discovered, its camouflage function deactivates once those viewing it pass through it.

The Sontarans used one during the Crimean War in 1855 near Sevastopol to hide their fleet of Sontaran flagships from the Light Division. The Thirteenth Doctor and Mary Seacole discover the shield after following Svild and subsequently the entire fleet. (TV: The Halloween Apocalypse)