UTDR and Fallen London
(spoilers for Deltarune ch5. Vague spoilers for some Fallen London lore and stories.)
I keep thinking about all the ties between Toby's games and the Fallen London setting.
Plot point of someone who disappeared without a trace, but left eldritch clues pointing to his continued existence (Mr. Eaten and Gaster. The man behind the tree is also a little like this.)
Out-of-the-way storyline about willfully choosing a destructive and clearly ill-advised path, with the potential to do real, irrevocable harm if one persists long enough (Seeking and No Mercy/Weird Route, though the "irrevocable harm" part hasn't fully panned out yet. Uncovering Secrets Framed in Gold is also kind of Weird Route like.)
Game set underground.
Themes of darkness and light. Ralsei's monologue in chapter 3 (roughly) about how darkness makes the laws of reality looser syncs very well with Fallen London lore about light enforcing the laws of reality.
Eldritch abomination Man Behind the Man furry called December who wants to fill the world with darkness.
Themes of water and drowning. Sunless Sea/Seeking use these themes heavily. Deltarune has the Sword Route, where you pursue eldritch secrets by sailing to a mysterious door in the North, and the ch5 Weird Route, where you walk into a lake.
Maybe other stuff I'm forgetting.
I'd be willing to put money on Toby having been inspired by Failbetter's stuff to some extent.
