Added to main post, my first draft of the Tiefling writeup.
What I've done is rewrote the Tome of Foes subraces as separate legacies and added a few others. I've also added 4 alternate abilties, for flight, natural armour, natural attacks and perhensile tail. Though the last one might need some more work.
Tieflings
Tieflings get their lineage from the Lower Planes which may include the influence of various fiendish creatures like Devils, Demons, Yugoloths, Gehreleths/Demodands, Night Hags, Hordelings, Bladelings and others or the malignant influence of such planes. Because of the fiendish nature of their lineage, Tieflings are often mistrusted by many who fear Tieflings carry a malignant influence. When invoking their fiendish powers some of that malignant influence might come to the forefront, as the fiend rattles it's metaphorical chains. In the struggle of their dual-natures, most Tieflings learned long ago to control the fiend inside them.
As a result many Tieflings are loners and often pushed to the fringes of society, due to the prejudices of many mortals. As a result many Tieflings have become self-reliant and independent, having to do their best to push their way through by society by sheer force of will. The fiendish nature of Tieflings does not inherently exert a predilection to malevolence, however how Tieflings are treated by the society they're raised in often does. Often Tieflings do band together as clans, extended families or gangs, and are often mistrusting of others, including other Tieflings.
In planar cities Tieflings are common sight, in many cases gathering together in often the poorer and less hospitable districts. Many Tieflings as outsiders often won't find a place among common society, often becoming thieves, gang members, harlots and con artists. Some Tieflings are driven even further outside into to joining cults or becoming adventurers. Many Tieflings do live their lives in poverty because of their place in society, though some Tieflings have made their place in society filled with wealth and privilege. It's often whispered in hushed tones that those Tieflings in wealth and power got their fortunes through nefarious means.
Outside of most planar societies, there's rumours of Tieflings enjoying a higher standing in some nations that are ruled by fiendish forces. These Tieflings often embrace their more malevolent natures as a result. There's said to be an empire on the Material Plane who's corrupt once Human rulers made a pact to the Lord of Nine Asmodeus, who cursed the entire nation turning everybody into a Tiefling.
Thanks to their natures some Tieflings can survive on consuming ashes or minerals for sustenance temporarily. Tieflings are more likely to eat meat, some preferring raw meat, and many do eat insects (some Humans do too).
Tieflings often have features like horns of varying sizes, tails, sharp teeth or cloven feet. Many Tieflings don't have any such features appearing mostly as Humans or other mortal races, and only containing a hint of a fiendish nature such as an ominous shadow, a slight scent of sulphur or a terrifying reflection in the mirror.
Characters
Ability scores & classes: Many Tieflings tend to be schemers and adept at larceny and swindling, thus many do put their ability bonuses in Charisma, Intelligence or Dexterity. Some Tiefling legacies embrace rage and other feral natures.
Rogues are a common class for Tieflings, as many of them do come from criminal backgrounds and it's something that easy for a Tiefling to embrace. Because of the nature of their soul and status as outsiders, many Tieflings confront their dual natures creatively through music and poetry becoming Bards. The power of the arcane comes naturally to Tieflings as many do become Wizards applying their understanding of their natures to the studies of magic. Many do embrace and seek to understand their Fiendish natures by becoming Warlocks with the Fiend Patron, but many Tiefling Warlocks seek other types of patrons to shield themselves from Fiendish influences.
Age: Most Tieflings age at the same rate as Humans or their mortal ancestry does. Some Tieflings might be long-lived or ageless.
Size: Most Tieflings are medium size. Some Tieflings are small size.
Speed: Tieflings have a walking speed of 30 feet
Type: Tieflings are Humanoid (I pondered for a moment the Humanoid and Fiend creature type, but it's more of a disadvantage to be both)
Languages: Tieflings understand Common and one other language, often one of their mortal ancestries or Abyssal or Infernal.
Darkvision: Tieflings have Darkvision of 60 feet
Fiendish Legacy: While there some common varieties, the powers of a Tiefling can be varied as the appearances of Tieflings. The manifestation of powers in a Tieflings legacy awakens a bit of a Tiefling's nature, internally it can be as much as a dark whisper they can hear in their mind. For some Tieflings they momentarily appear more fiendish when invoking such powers. Many Tieflings describe using their legacies as Invoking the Fiend. According to some certain legacies can colour a Tiefling's temperament.
For legacies, Tieflings typically known one Cantrip at the the start, at 3rd level they know one 1st level spell that's enhanced to a 2nd level spell slot if possible and can be cast once before requiring a long rest (though some might be usable with a short rest), and at 5th level they gain knowledge of a 2nd level spell that can be cast once before requiring a long rest to cast again. Charisma or Intelligence is the spellcasting ability for spells cast through this ability (Also see the optional rule Innate spell known).
Optional Rule: Custom Legacies - See above for the general rule most legacies follow.
Legacy of the Foreboding - Menace and dread are the flavours invoked by this Legacy, an often common one where the fiend inside exudes malevolence and spite. It's thought that this might be the most common Tiefling legacy (it's the default one). The powers of this legacy are: Thaumaturgy, at 3rd level Hellish Rebuke cast as 2nd-level spell once before requiring a long rest, and at 5th level Darkness cast once before requiring a long rest. (Formerly Asmodeus)
Legacy of Corruption - Twisting and poisoning things is what the fiend inside desires, it seeks to weaken others to impose its will on the outside world. The powers of this legacy are: Thaumaturgy, at 3rd level Ray of Sickness cast as 2nd-level spell once before requiring a long rest, and at 5th level Crown of Madness cast once before requiring a long rest. (Formerly Baalzebul)
Legacy of the Infiltrator - The fiend inside seeks to learn the darkest of secrets to exert control and destroy things from the inside. It's often mistrusting of others. The powers of this legacy are: Thaumaturgy, at 3rd level Disguise Self once before requiring a long rest, and at 5th level Detect Thoughts cast once before requiring a long rest. (Formerly Dispater)
Legacy of the Tempter - The fiend likes to bend others to its will through the force of personality, it's better to convince others to work in your agenda than to do such things yourself. The powers of this legacy are: Friends, at 3rd level Charm Person once before requiring a long rest, and at 5th level Suggestion cast once before requiring a long rest. (Formerly Fierna)
Legacy of Deception - The fiend desires to bend or hide the truth, lies become its weapons as the truth itself is something that's malleable under deft control of one such as itself. The powers of this legacy are: Minor Illusion, at 3rd level Disguise Self once before requiring a long rest, and at 5th level Invisibility cast once before requiring a long rest. (Formerly Glasya)
Legacy of Isolation - The fiend is cold-hearted and uncaring, it depises others wishing nothing more than pain and ruin on those it feels have violated it's sancutary. The powers of this legacy are: Ray of Frost, at 3rd level Armor of Agathys cast as 2nd-level spell once before requiring a long rest, and at 5th level Darkness cast once before requiring a long rest. (Formerly Levistus)
Legacy of Avarice - The fiend inside seeks to gain and horde all that it can it's own means, it jealously hides what it can away from others. The powers of this legacy are: Mage Hand, at 3rd level Tenser's Floating Disk once before requiring a short or long rest, and at 5th level Arcane Lock cast once before requiring a long rest. (Formerly Mammon)
Legacy of Malice - The fiend would rather have the world outside burn, everything is to be consumed and nothing left but ashes to rule over. The powers of this legacy are: Mage Hand, at 3rd level Burning Hands cast as 2nd-level spell once before requiring a long rest, and at 5th level Flame Blade cast once before requiring a long rest. (Formerly Mephistopheles)
Legacy of Wrath - The fiend inside is driven by bloodlust and rage seeking nothing more than conflict to sate its thirst. Visecral physical violence is often its means. The powers of this legacy are: Thaumatury, at 3rd level Searing Smite cast as 2nd-level spell once before requiring a long rest, and at 5th level Branding Smite cast once before requiring a long rest. (Formerly Zariel)
Legacy of Madness - Existence is pain, and sanity is madness, and nothing means what most think it does to this fiend inside. Twisting minds to see things it's way is its desire. The powers of this legacy are: Viscious Mockery, at 3rd level Tasha's Hideous Laughter once before requiring a long rest, and at 5th level Crown of Madness cast once before requiring a long rest.
Legacy of Decay - Entropy is course of all things, everything is dying from the inside and the fiend seeks to speed up the process. With it's touch it'll set things right towards the path of destruction. The powers of this legacy are: Acid Spray, at 3rd level Tasha's Caustic Brew (Tasha's Cauldron of Everything) cast as 2nd-level spell once before requiring a long rest, and at 5th level Melf's Acid Arrow cast once before requiring a long rest.
Legacy of Paranoia - The fiend seeks to survive at all costs, no deed is beyond it whether it needs to lash to or to stay away, as it sees that everything is against it. The powers of this legacy are: Chill Touch, at 3rd level False Life cast as 2nd-level spell once before requiring a long rest, and at 5th level Invisibility cast once before requiring a long rest.
Optional Rule: Alternate spells - At character creation you may decide to replace a single spell from a legacy with another spell or cantrip of the same level. This rule can be in place instead of allowing custom legacies.
Fiendish Resistance: A Tieflings nature often makes them resistant to the affect of certain elements. Pick one of the following damage types: Lightning, Fire, Cold, Acid or Poison.
Alternative Abilities:
Some of these abilities might be restricted at character creation, by the DM's descretion. They might be obtainable later with the Planar Legacy feat.
Fiendish Wings - The Tiefling has a working pair of bat-like or darkly feathered wings. They gain a fly speed of 30 feet. This ability replaces Fiendish Legacy.
Alternate rule for Fiendish Wings - A Tiefling with wings gains the ability to glide, subtract 100 feet from a fall when determining damage taken, and they can move 2 feet horizontally for every foot they fallen. At 5th level they can fly with speed of 30 feet.
Fiendish Hide - The Tiefling has a tough hide, one that might be scaly or covered in spikes or small ridges. They have an Armor Class of 13 + Dexterity modifier when not wearing armor. This replaces Fiendish Resistance.
Natural Weapons - The Tiefling has one of the following natural attacks. Claws - 1d4 slashing and target is poisoned for until the next round. Stinger 1d4 piercing and the target is poisoned until the next round. Horns 1d6 piercing and the target is moved 5 feet away. This ability replaces Fiendish Legacy.
Prehensile Tail - The Tiefling has a tail can grasp and manipulate objects. They can hold but not attack with any tiny object, and can draw or stow an additional object on their turn. They also get a +2 bonus using, Thievery (Dexterity) for checks involving sleight of hand or Atheletics (Strength) checks involving climbing. This replaces Fiendish Resistance.
