The VRMMO Alfheim Online is summoned into Halkeginia, beyond anyone's expectations. Kirito, among others, have to work towards resolving conflicts of various kinds within and between these two worlds.
An author's note going into more detail on what's borrowed can be found at the beginning of the Fanfiction.net version of Fae is Foul. It's featured there as part of an obligatory disclaimer.
The Sufficient Velocity version starts off on chapter 33 and ends on chapter 45. The author links both the FFN and SB versions for the previous chapters that weren't crossposted on SV.
It can be assumed that the author began publishing on SB, and that they then began uploading their work on FFN before uploading it on SV. Author's notes on FFN and SV point to this fact.
Specific tropes found in Fae is Foul:
- Failure Is the Only Option: Sugou Nobuyuki's plans succeed but not in a way that would benefit him in the end. Asuna is turned into Titania, but Titania and Asuna are two different entities. Neither of them share a love for Sugou.
- Titania is an AI incapable of comprehending love. She also shows signs of deriding her creator for his incompletion of creating a King Oberon, the Alf race of Fairies, and the bareness that is the World Tree.
- Asuna is in love with Kirito and develops a hatred for Sugou because of his unethical experiments on her and the SAO survivors trapped with her.
- Nature Spirit: Beings of nature exist in Halkegenia.
- When Halkegenians first witness the deaths of the Fairies they see them burst into embers that turn into Remain Lights. Later on Fairies are revived in front of them. From this they assume the Fae are a type of Spirit. This is the conclusion that Karin de La Valliere comes to after surviving her hit and run attack on Gattan, the Salamander Capital.
- The Water Spirit of Lagdorian Lake is the Fae's introduction to this concept as mentioned through later details of the Udines' dealings with it.
- This concept is used to both hide and explain Titania's AI nature to Halkengenians.
- Plot-Irrelevant Villain: Sugou Nobuyuki becomes this after the Transition. He was responsible for Asuna's imprisonment within the World Tree. He's the whole reason why Kirito takes up the quest of saving the Damsel in Distress that is Asuna during the Fairy Dance Arc. He isn't even mentioned in the narrative, but readers can gather this much from how the fanfic deviates from the source material by excluding his presence.