Nice to see Karn again. The Brain of Morbius, the Fourth Doctor serial in which it first appeared, is one of my favourites. Steven Moffat first resurrected the planet Karn and the Sisterhood for the 50th anniversary prelude The Night of the Doctor.
So now it's time to speculate on who they're talking about in the new video. We can assume it's not the Master. Maybe the Brigadier? Morbius? Someone completely new? Peter Capaldi's character in "The Fires of Pompeii"? Jack Harkness? Santa Claus? Really it could be anyone, there aren't enough clues.
I only had time to listen to one audio play this week, a Fourth Doctor Lost Story--that is, part of a series from Big Finish where they record adapted scripts from the old series that were never produced for television for one reason or another. I listened to Foe from the Future, adapted from a script by Robert Banks Stewart who wrote Terror of the Zygons and The Seeds of Doom. The Foe from the Future features Louise Jameson as Leela and Tom Baker reprises his role as the Fourth Doctor.
I don't know why this script was never produced for television unless it's the extreme violence. It features a lot of grotesque mutation and cannibalism but The Brain of Morbius featured stitched corpses, The Seeds of Doom had people ground up in a large machine. I guess none of it was as graphic as The Foe from the Future would have needed to be.
It begins like a Scooby Doo story with the Doctor and Leela investigating ghost sightings in Devon in the late 1970s. The ghosts turn out to be holographic projections, of course. They're caused by a society in the future trying to build a bridge to the 20th century to take refuge from some giant insect menace. There's a lot of fun had parodying this future society's attempt to imitate the 20th century--for some reason they think Bruce Forsyth was Prime Minister. Then things get weird and horrifying. It's a good story.