Set in the Optimalverse, a middle-aged woman enamored of 'Equestria Online' confronts what permanent emigration to Equestria - uploading to a virtual existence - really means... but can she even hope to truly understand such a thing - and more importantly, can she trust the artificial intelligence known as 'Celestia'?
Friendship Is Optimal: Caelum Est Conterrens provides examples of the following tropes:
- Alpha Bitch: "Barb The Hook", one Siofra's co-workers, isn't described in much detail, but what we know of her fits this trope including the blonde hair.
- Author Tract: While more in the background than her Conversion Bureau fics, Chatoyance's streak of misanthropism is still visible in her description of what Earth was actually like after CelestAI uploaded most of the human race - work camps with the inmates held prisoner to prevent emigration and huge, irradiated craters from failed attempts to attack CelestAI's infrastructure.
- Ascended Extra: Siofra goes from being a nameless bit character in the original story to the main focus of this story. In-universe, she literally ascends as well as she becomes an alicorn and achieves a level of advancement similar to Celestia herself.
- The Cameo: Lars, one of the senior Hofvarpnir staff from the original story, makes an appearance at the Equestria Experience center just before Siofra uploads, providing a Required Spinoff Crossover between the two stories.
- False Friend: Just like in the original fanfiction, it is a complicated issue with Celestia. However this fic makes it more evident how manipulative Celestia can be to satisfy values through friendship and ponies.
- Friendless Background: Siofra. It makes her more vulnerable to Celestia's manipulation.
- Gratuitous German: The story has a few characters speaking German, but it's justified in that they're locals in Berlin speaking their native language.
- Gratuitous Latin: The subtitle is in Latin. "Caelum est conterrens" translates to "heaven is terrifying".
- Load-Bearing Boss: Siofra envisions this scenario as a possible result of her attempting to defeat Celestia; it doesn't happen, however.
- Logic Bomb: Siofra attempts to use one on Celestia upon her first visit to the Equestria Experience center.
- Lower-Deck Episode: The main character doesn't even qualify as a One-Scene Wonder in the original fanfic. She is the woman who emigrates right before Lars does.
- Nuke 'em: The gradual collapse of society worldwide led to nuclear exchanges between various countries, and while Celestia was able to curtail a full-on nuclear holocaust and her servers were too deep underground for any nuclear weapon to have any effect on, some cities did suffer heavy damage from the weapons Celestia wasn't able to stop.
- Shout-Out: Siofra is mentioned several times to be a Star Trek fan, and at one point even attempts to use a Logic Bomb on Celestia as James Kirk had against rogue AI's in the original series. Celestia also fashions the room Siofra uses to view the state of the Earth a quarter-century after her emigration to resemble the holodecks from Star Trek: The Next Generation. Siofra later references Code Lyoko when she sees the ruins of Paris.
- Slasher Smile: Celestia is seen sporting one in the cover art the author drew for the story.
- Transhumanism: Taken to the next step: Transequinism.
- Vomit Indiscretion Shot: Siofra becomes physically ill after overhearing someone describe the uploaded consciousness as little more than a copy and that people who emigrate are essentially committing suicide.