First, a clip from Doctor Who.
This is from the Seventh Doctor episode “Battlefield”, aka the episode where Brigadier Lethbridge-Stewart finally gets to shoot a threat (there’s a line in the Third’s period where the Brigadier wishes for once he could meet an enemy that wasn’t immune to bullets) and win in one of my favorite moments of the classic series. It took a special bullet mind you but it’s a great farewell to the character as he never got to appear again in the series classic or new, just an episode of The Sarah Jane Adventures before Nicholas Courtney’s passing, and some audio dramas with a new performer. I’m bringing this episode up to ask just when does the Doctor become Merlin? Morgaine (stand-in for Morgan Le Fay) also refers to him as Merlin and the Doctor tells Ace that maybe someday he will be.
Now, outside of forgetting an adventure, when would the Doctor get to travel to the Camelotian dimension and become Merlin? My best guess is the War Doctor, after his using the Moment to end Galifrey, or perhaps this got screwed up in the timeline when the Moment called in two later Doctors to alter the end of the Time War, the War Doctor goes on something of a pilgrimage to re-find himself before actually regenerating into the Ninth Doctor. Going through other official adventures, knowing or assuming the people he meets in Camelot saw him regenerate as some comments imply, and knowing it has to happen after this so the Doctor/Merlin can tell Ancelyn about the events of “Battlefield”, it’s the only place that really fits…in the same way of trying to explain Jaime and the Second Doctor helping the Time Lords in “The Two Doctors”, but it’s still a possibility.
Here’s a truncated version of the episode, from the official Doctor Who YouTube channel.