The Doctor (Battlefield)

An incarnation of the Doctor was known at various times as Merlin, (TV: Battlefield, PROSE: Battlefield, Happy Endings) Muldwych, (PROSE: Birthright, Happy Endings) Dokien, (PROSE: The Collection) and the Flying Dutchman. (PROSE: Transit)

A day to come
When the Seventh Doctor encountered the knights Ancelyn and Mordred from another dimension, they both recognised him as Merlin, (TV: Battlefield, PROSE: Battlefield) saying that while his face was different, the life behind his eyes was unmistakable. (PROSE: Battlefield) Guided by instructions left by Merlin, the Doctor found Excalibur and defeated Morgaine and Mordred. (TV: Battlefield, PROSE: Battlefield) The Seventh Doctor later met this incarnation several times, both on Antýkhon (PROSE: Birthright) and at Bernice Summerfield's wedding to Jason Kane. (PROSE: Happy Endings)

In a Tomorrow Window, the Eighth Doctor saw a "short, impudent-looking man" with ginger hair and an afghan coat. (PROSE: The Tomorrow Windows) Shortly after their regenerations, both the Tenth and Eleventh Doctors expressed dismay about not being ginger. (TV: The Christmas Invasion, The Eleventh Hour)

Arthur's World
For some time, this Doctor resided in Arthur's World under the name "Merlin". There, he fought Morgaine on several occasions. Once, after she bound him in ice in Breceliande, the Doctor summoned a dragon to melt it. Songs were sung among the people of the Doctor's "power over light, darkness and the elementals".

Merlin taught King Arthur since he was a child, and saved his life on many occasions. When Arthur was old and had almost lost the battle to Morgaine, Merlin spoke with him of how the battle would end. Merlin took Excalibur from him and sent it back in time so that his seventh incarnation would use it to defeat Morgaine. (PROSE: Battlefield)

Merlin later sent Excalibur to Winifred Bambera and Ancelyn so that they could prevent the Deindum from causing Phobos and Deimos to break away from their orbit of Mars. Because he needed to be "somewhere else", he used a hologram to speak to Bambera and Bernice Summerfield when they reached the chamber in which Excalibur could be used. (PROSE: Excalibur of Mars)

Traveling
This incarnation of the Doctor gained notoriety as a data pirate called the "Flying Dutchman". Francine tried to track him down in the datascape, but she only saw him once. From his digital three-masted galleon, crewed by cartoon skeletons in striped jerseys, he gave her a Braille document that led her to investigate Imogen's attempt to create a second generation of ubersoldaten. (PROSE: Transit)

In 21st century Exeter, Tegan Jovanka encountered this incarnation of the Doctor and his companion Anna. Tegan wrote about these events in a book called Good Companions, which Anna found and read in the TARDIS library; since it warned about the Doctor manipulating his companions into killers, Anna asked the Doctor to take her home, which he reluctantly did. (PROSE: Good Companions)

While traveling with another companion, Guin, the Time Lords sent the Doctor to resolve a temporal echo at the Schrödinger Institute research station. Because of the echo, the Doctor and Guin were trapped in a repeating time loop, unable to be seen or touched by any of the scientists. Furthermore, they could only touch move objects after one of the scientists did it. After being trapped in a few cycles of the loop, the Doctor found a control box that a technician had worked on; by rewiring it, the Doctor broke the echo. However, the scientists and the station faded away as they left. (PROSE: Revenants)

Travelling alone, the Doctor went to visit his old friend Greenaway, who had been in a coma for many years. None of the Doctor's attempts to awaken Greenaway had succeeded, so the Doctor turned off his life support machine in an act of mercy before leaving in the TARDIS, unaware that turning off the machine had finally shocked Greenaway out of his coma. (PROSE: Greenaway)

Antýkhon
After losing his TARDIS, the Doctor became trapped on Earth sometime before the year 16909, when the planet was known as Antýkhon and inhabited by Hairies. He built a hut at the top of Mount Kukúrk in Africa and became known as "Muldwych". He lived there for over 500 years.

The Charrl came to Antýkhon in the hopes of establishing a new homeworld but then became trapped on the hot and polluted surface. At Queen Ch'tizz's request, Muldwych helped them make a temporal link to a more habitable time in Earth's past, where he knew it would be easier for him to escape. Through the Great Divide, the Charrl formed a link with Russia in 1908, but it was unstable, fatally injuring any travellers.

Seven months later, the Divide moved to London's East End in 1909. The Seventh Doctor then encountered Muldwych and, despite his concerns about the First Law of Time, he agreed to help by splitting his TARDIS in half and sending the parts up each half of the Divide. After half landed on Antýkhon, Muldwych retrieved it and travelled with Ace, Seeba, and Korin to Hanbury Street, where they met Bernice Summerfield. Reuniting the two halves of the TARDIS, Muldwych stabilised the connection; however, since the Charrl couldn't invade that part of Earth's history, Muldwych promised he would lead them to an alternate Earth, then trapped them in an unused section of the Seventh Doctor's TARDIS. He then tried to use that TARDIS to escape Earth, but it instead returned him to Antýkhon.

Shortly afterwards, the Seventh Doctor visited Muldwych and shared a cup of tea with him. Muldwych put whisky in his tea. Remembering Ace's eventual fate, Muldwych gave him the novel Madame Bovary as a present for her. (PROSE: Birthright)

Muldwych was eventually able to escape by using "a narrow beam of chronons perfectly aligned with the marginal reality established by the Puterspace apparatus" used by Time, Pain, and Death to visit Bernice Summerfield and Jason Kane's wedding. Along the beam, he visited another marginal reality, where he met and befriended a unicorn named Bat, before himself arriving at the wedding. At one point, he talked with Blake and Hamlet Macbeth about the origin of Excalibur.

At the wedding, Muldwych gave the Seventh Doctor the book The Unformed Heart and two halves of a time ring to use as the wedding rings. (PROSE: Happy Endings) Using the time ring, Jason and Benny themselves were stranded on Antýkhon and helped Ch'tizz and the Charrl without Muldwych's help. (AUDIO: Birthright) Later, "Dokien" visited the Braxiatel Collection, rescued Jason Kane from the past by bringing him back to the Collection, and then left with Benny and Jason's wedding rings as a momento. Jason was indignant at this, but Braxiatel told him that Dokien was the one who gave them the rings in the first place. (PROSE: The Collection)

Appearance
He was very short, (PROSE: Battlefield, Revenants, The Tomorrow Windows) with a plump figure and a ruddy face. (PROSE: Birthright) He had curly ginger hair, (PROSE: Battlefield, Revenants, The Tomorrow Windows) though later in his life, it was grey with streaks of red (PROSE: Happy Endings) or grey-brown. (PROSE: Birthright)

At different times, he was seen wearing a felt hat and an afghan coat; (PROSE: Transit) the afghan coat over a mustard yellow waistcoat, a crumpled white dress shirt, open toed sandals and socks, and bracelets; (PROSE: Good Companions, Revenants) and a white robe. (PROSE: Happy Endings) He wore a blue Roman-style ring on his left hand (PROSE: Birthright) and had a strong scent of cologne. (PROSE: Greenaway) At one point, he wore an eyepatch. (PROSE: Excalibur of Mars)

Behind the scenes

 * This incarnation of the Doctor isn't the only one to have been known as Merlin: the Eighth Doctor took over the role in One Fateful Knight. Fugitive shows the Tenth Doctor to have later claimed that he was once called Merlin by King Arthur, with no word on which of the two incidents he was recalling, if it wasn't yet another instance of his adopting such an alias.
 * Peter Anghelides said he partly based this incarnation on one of his university friends.
 * A future ginger-haired incarnation of the Doctor was set to appear in Doctor Who: The Last Regeneration by Lee Sullivan, but the comic was cancelled.