Came across this post by checking the Doctor Who hashtag on Mastodon and replied there. Doesn’t look like it got back to you; so, since I’ve been meaning to look into making a Lemmy account I’ll try this.
That’ll be the Dark Tower. AKA Rassilon’s Tower. The Tomb of Rassilon.
Might be a fediverse quirk of not-quite seamless interconnection between systems. But since this is my second post on Lemmy I’ll take the opportunity to try links and spoilers. The Mastodon post is here.
Looking at your context, you’ll have encountered the main NuWho incarnation of Rassilon in
spoiler
The End Of Time, 10’s regeneration story. He’s the big Time Lord cheese in that.spoiler
then you’ll have seen a somewhat diminished incarnation of him as the Lord President that 12 deposes in Hell Bent.But there’s a lot of lore out there. A lot. And if you weren’t already familiar with Rassilon before his appearance in NuWho then his name in those moments could easily slip past.