The Master's masks

The renegade Time Lord known as "the Master" was known to wear elaborate disguises, which often included masks which covered his face. (TV: Terror of the Autons, et al.) The Master was also known to create similar masks to put on other figures to disguise them as himself. (TV: Terror of the Autons, GAME: Destiny of the Doctors)

According to himself, the masks were in fact a part of his TARDIS, thus explaining their talent at imitating other's appearances. The masks would obey his mental commands, thus allowing him to use it to attack others as well as form such things as parachutes.

The Master's masks and false identities were so well known that UNIT kept several codewords for events where one person recognised the Master's true identity, including terms as elaborate as "special security protocols" or as simple as "Code Magister". At one point, after being recognised by two different people in one day, the Master noted that the masks were in need of retirement, and asked if there was a UNIT museum. (COMIC: The Heralds of Destruction)

The Fourth Doctor felt that "No matter how intricately [he would] disguise [his] feaures, [the Master's] true emaciated form [wasn't] far below the surface." (AUDIO: The Evil One)

History
The Master first used one of his masks to disguise himself as telephone mechanic. He later put a mask on Rex Farrel, disguising Rex as himself and causing Mike Yates to shoot Rex in self-defence. (TV: Terror of the Autons)

The Master later used the disguise to sneak onto a UNIT base to study the attacking robots, disguised as General Mayhew. The Brigadier was able to discover that the figure wasn't really Mayhew by inventing an element of the general's life (his wife, who he fictitiously named Beatrix) and then calling out the element as false once the Master went along with the lie. The Master was quickly revealed, as was arrested by the Brigadier.

In a helicopter ride to the location where he was to be held prisoner, the Master revealed the details of the mask to the Brigadier, and telepathically used it to attack him. The Master then used the mask to escape, expanding it to serve as a parachute.

The Master then arrived at UNIT HQ while the Doctor and Jo Grant were in a deep trance, and knocked out the Brigadier, using the mask to take his place. Once the Doctor awoke from this, however, he immediately recognised that Brigadier in front of him was a fake.

After breaking into Electronicon Ltd., the Master used the mask to create an imitation of one of the labcoat scientists which he swiftly killed. Mike Yates used this mask to intercept the main room of the building.

When stuck in 1868, the Master used the mask to camouflage their secured Helicopter. Later he used it to summon his TARDIS in order to make his escape. (COMIC: The Heralds of Destruction)

For some time, the Master wore a bejewelled mask following his loss of Tremas's body. (AUDIO: Dust Breeding)