How flexible is a Tiefling tail, and How much control do they have over it?
What real world animal has a tail most similar to a Tiefling in terms of control, and not look?
The books published so far (for 5e, at least) seem pretty thin on description of tieflings in the detail you want. By searching, the most I could find that referenced the tiefling's tail is the default description given in the Player's Handbook (pg 42):
They have thick tails, four to tive feet long, which lash or coil around their legs when they get upset or nervous.
And this part of the Variant Tiefling option in the Sword Coast Adventurer's Guide (page 118):
Your tiefling might not look like other tieflings. Rather than having the physical characteristics described in the Player's Handbook, choose 1d4 + 1 of the following features: ... a forked tail...
And there's also the official art. With those depictions in mind, here's my take on the tiefling's tail:
The official art of tieflings from the manuals gives them quite chunky tails indeed - the character illustration for tieflings in the PHB depicts a character whose tail looks to be thicker than her neck at the base and is at least as thick as her arm for most of that 4-5ft length. It is displaying an appreciable curvature, though, which suggests a decent degree of flexibility. The flavour given that a tiefling's tail may lash about or coil around their legs also suggests to us that it has a reasonable range of motion. It would seem reasonable, based on the art and the description, to describe the tail as being about as a flexible as a snake with similar dimensions.
The question of how much control the tiefling has over their tail is a bit trickier. The only example of tail behaviour we're given is based on the tiefling's emotional state; a tail that lashes or coils, sounding a lot like a cat's does, without deliberate control. We can also extrapolate from what is not stated - the tiefling has no special racial trait which explicitly lets them make use of their tail for a functional purpose. That suggests that whatever control they do have over the appendage, it is not fine enough that they can interact with their environment using the tail anywhere near as adeptly as their other limbs.
Based on that, I'd judge that the best real-world animal to use as an example would actually be a cat. Cats' tails are flexible and can lash and coil like the tiefling's is described as doing - and a cat may deliberately flick their tail about sometimes, and tail behaviour is a helpful indicator as to a cat's mood - but they aren't capable of using their tail to pick things up, or manipulate their environment beyond hitting things (relatively gently).
A tiefling is more intelligent than your average cat, so you could imagine they could be a bit more creative in the use of their tail. They could probably use it to pick up reasonably sized objects that are both sturdy enough to withstand a bit of rough handling but not too heavy; potions and scrolls seem too small for the tail to be able to coil around enough to grasp, but I'd judge a spear/polearm or a bucket could be lifted (but not manipulated effectively) if the tiefling concentrates on what they're doing.
Ultimately I would probably rule that tiefling could deliberately flick/lash their tail about, or coil it around something reasonably sized and stationary, using the free object/environment interaction they get on their turn. That means they could do stuff like flick levers, behave inappropriately in bars, and knock your drink off the table with relative ease. I'd also suggest that a tiefling spending their action, and therefore concentrating their effort, on controlling their tail could use it to pick up reasonably sized objects, but could not manipulate them adroitly enough to do anything effective with them beyond carry them (no making attacks with a weapon held in the tail, etc.)
The 5th edition Player's Handbook describes the tiefling's tail as follows:
They have thick tails, four to five feet long, which lash or coil around their legs when they get upset or nervous.
They cannot wield a weapon or pick up items with their tail, or the game rules would specify as such. They cannot attack with it. Nor does it give any bonus to balance. This lack of tail-specific rules defines the limits of the tiefling tail's flexibility.
In earlier editions, tieflings varied considerably in appearance. However, in D&D 5th edition, the core tiefling is of that specific type, bearing the blood of Asmodeus after an ancient pact generations ago, and all have similar tails.
This is true even in the Forgotten Realms. Historically, most tieflings in the Realms were the descendents of extraplanar visitors and had great variety in their features. During the Spellplague, all tieflings were converted to the Asmodeus variety (Sword Coast Adventurer's Guide, p.118). However,
The tieflings descended of Asmodeus originated in D&D 4th edition. We can learn more about their tails by reading materials from that edition. A continuous canon exists between editions for at least the Forgotten Realms setting, so it's reasonable to consider earlier material for descriptive features.
In Races and Classes, Chris Sims describes their intentionally unified origin and appearance:
A common origin meant we could give tieflings a unified appearance, and that look could be edgy instead of ugly. This cohesive origin allows players to imagine what their individual tiefling is like, as they would a human, without worrying about a list of possible devilish traits. Further, knowing that every tiefling shares a similar body type makes it easier to write new material for tieflings.
The Player's Handbook describes that the tail is specifically nonprehensile. That means a tiefling can't pick things up with it.
They have large horns; thick, nonprehensile tails that range in length from 4 to 5 feet ...
Player's Handbook 2 describes tiefling tails as "sinewy" - thin, whip-like:
Each pair of horns, every sinewy tail, and every set of glowing red eyes reminds the world of the perils of dealing with devils and the evils that result from such fell bargains.
In both editions, the sources support the view that the tiefling's tail is non-prehensile, and able to lift no more than its own weight. It's closest to a cat's tail in terms of ability: it can lift, display emotion, and knock things off shelves, but not grasp, pick up, attack, or convey any real amount of force.
Since tieflings are humanoids and the real world examples of humonoids are strongly related to monkeys why wouldnt you assume it is like monkey tail. If you say the tail is inherited from the devilish ancestor i would think about the ancestors traits.
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