Trying something out, based on everything I've read so far... If you notice a mistake (e.g. using Ulvaerren as the daemonfey house that withdrew to the Abyss) or an omission (e.g. a power evidenced by a daemonfey NPC in a novel or book), please let me know. I've also cross-posted to Candlekeep, in case the sages there can offer insight.
Descending from sun elf houses that consorted with demons and succubi, the daemonfey were hunted down by elvenkind until only House Dlardrageth and its vassal houses remained in Faerûn. However, House Ulvaerren## fled to the Abyss where Graz’zt welcomed them with open arms, though at the cost of sacrificing their ability to plane shift. A daemonfey cambion or tiefling has the following changes:
Fey Ancestry. The cambion is considered an elf for purposes of all effects related to race. It has advantage on saving throws against being charmed, and can’t be put to sleep by magic.
It know either the friends or minor illusion cantrip.
It can speak, read, and write Elven.
Many daemonfey possess unique powers. You may replace a cambion’s fly speed and Fire Ray with one of these options:
Dark Call. When a creature fails its save against the daemonfey’s command to “approach”, each allied creature within 5 feet of it must make a saving throw against the command to “approach" as well.
Eyes of the Arcane. When the daemonfey casts detect magic, it learns the name of any spell on a creature it sees and if the creature can cast spells.
Skin Dance. When the daemonfey casts alter self it may appear as a specific humanoid it can see within 60 feet. Additionally, that humanoid must succeed a DC 14 Charisma saving throw or it is teleported, swapping places with the daemonfey. A DC 14 Intelligence (Investigation) check made as an action reveals which is the daemonfey.
The Cambion would be fine assuming the creature were the direct offspring of a fiend and a humanoid. They're the half-fiends of 5e. If the fiendish ancestor was... somewhere... back in the ancestral line, then you're really looking at a modified tiefling.
That said, if, as DM, you wanted the daemonfey to be a bit more than just a tiefling, you're well within your rights to base them on the cambion instead.
Tom Costa (Demihuman Deities & Races of Faerûn) recommended I take a closer look at the daemonfey stat blocks in 3e's Lords of Darkness. Holy heck, they are really unique! Very much a "descended from X demon" in much the same way that MToF has "tieflings descended from Y archdevil" and "special traits for cambions descended from Z demon lord":
Sarya Dlardrageth, whose dad was a balor, gets...
• Bat wings.
• Innate spells: At-will detect magic; 3/day affect normal fires, burning hands, fear, pyrotechnics.
• Immune to fire & poison.
• Vulnerable to cold.
• Heat Aura acting as permanent fire shield, and can concentrate to ignite flammables.
• A bunch of custom magic items.
Ryvvik Dlardrageth, whose dad was a vrock, gets...
• A lesser version of a vrock's screech.
• Vestigial underarm wings provide him with feather fall at will.
• Telepathy (with strange proviso that it requires line of sight).
• "Mind Surge" globe or ray which paralyzes 3/day.
• An item called a kiira gem which allows him to store all knowledge read from the minds of others.
Xhalh Dlardrageth, whose mom was a marilith, gets...
• Six arms.
• Innate spells: At-will detect magic; 1/day exhale cloudkill.
• Never surprised.
• Regeneration (slow, 1 hp per 10 minutes).
• Immune to poison & gas.
• A bunch of custom magic items.