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) and Dokien. (PROSE: The Collection)

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, 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 regenerating, both the Tenth and Eleventh Doctors complained 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
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. At Queen Ch'tizz's request, he helped the Charrl make a temporal link to a time in Earth's past where it would be more suitable for them to live and easier for him to escape. The Seventh Doctor helped Muldwych stabilise a connection to the Great Divide in London's East End in 1909; since the Charrl couldn't invade that part of Earth's history, Muldwych promised he would lead them to an alternate Earth, but he instead trapped them in an unused section of the Seventh Doctor's TARDIS. He then tried to use that TARDIS to escape Earth, but it sent him back to Antýkhon. (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 the rings to them 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) His curly hair was ginger, (PROSE: Battlefield, Revenants, The Tomorrow Windows) though during his exile, it seemed grey-brown. (PROSE: Birthright) Later in his life, his hair was mostly grey with streaks of red. (PROSE: Happy Endings)

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)

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, and in Fugitive the Tenth Doctor later claimed that he was once called Merlin by King Arthur.
 * Peter Anghelides said he partly based this incarnation on one of his university friends.
 * A ginger-haired future incarnation of the Doctor was set to appear in Doctor Who: The Last Regeneration by Lee Sullivan, but the comic was cancelled.