I mean tails that can be used not only for extra balance and climbing (the way some monkeys do) but also used to grasp, carry and manipulate objects.

So the tail becomes a slightly less effective hand but can essentially do everything a hand can do, as well as giving the Tiefling unnatural balance and climbing abilities (at least when compared to those races without tails)

Good idea or not?

It doesn't really add much to them. Personally the artistic design of the tail doesn't make me think of that, nor is there a natural rationalization for it.

Do demons have this trait? etc. I don't know enough to say if it makes sense there.

There was a Talent like this in 4th that you could choose when you where lvl6 or something ( and a tiefling ofc), where you could use your tail to stash/retrieve items from your inventory, and hold items, not attack though.

A classic would be a tail swipe, to make a mob prone with a CMD check or something, then a Once per turn, where you can stash/get an item from your inventory, as long that it is accesible and makes sense.

I don't think it's a bad idea or gamebreaking so long as you do so within reason. The tail shouldn't be a third arm. If you had need for a teifling to have a prehensile tail you could effectively duplicate the rules for Loxodon from Ravnica. A prehensile appendage that's good for basic movements, but lacks the finesse and digits of an arm/hand.

I see what you are saying, bit that is what I meant - a less efficient hand.  So they can do things with a tail that they can do with their hands but they get disadvantages when using their tail for any intricate stuff,because well - it is a tail.

I am.thinking along the lines of things like spider monkeys and things.

These kinds of monkeys can carry, grasp and manipulate things with their tails and can even use their tails along with tools in a limited fashion.

In addition, their tails often have an exposed tactile.pad at the end that the monkey can use to "feel" with.  Primarily this is used for a pressure sensor, to tell the monkey how hard it's grasping into something, but it can also be used to feel or "sense" the world.

Like when these mo keys are climbing, their tails can sometimes seem to seek out and grasp onto branches of their own accord.  In reality though, the monkey is "feeling" with its tail 

They also use their tails like a rudder, helping to steer and balance them when performing dextrous tasks like acrobatics.

This is the kind of thing I am thinking of giving to Tieflings in my world.  They can grasp, carry, manipulate things and the world with their tail in the same fashion as real world monkeys who have prehensile tails.

I also wanted to give them the ability to do tasks their their tail but impose disadvantage on the tail if it tries to do anyrhing that really does require fingers.

There is a lot a Tiefling could do with their tails though that wouldn't have disadvantage.  So their tails become an extension of their bodies (an actual appendage that has uses) and not just an artistic flare.

What I am.unsure is though, if this would give Tieflings too much advantage over and make them too over powered when compared with races that don't have tails lack.a prehensile tail (like Lizardman has a tail, but it isn't a perhensile one as a Lizardman uses his tail to help him swim through the swamps of his home in much the same.way as a real world.alagator or crocodile might.)

I had created a hombrew prehensile tail feat for tieflings, modeled on the elephant-type race (trunk) from GGTR.

https://www.dndbeyond.com/characters/feats/53967-prehensile-tail

I am a huge fan of prehensile tails, so I'd endorse this enthusiastically.

As for the possibility of causing the character to be overpowered in relation to non-prehensile-tail characters; that may well depend on how well the players can RP that tail. I've been in games where players had a specific ability that could be overpowering, but did not utilize it to the fullest potential, so it was considered a useless ability, until someone else played the same trait to it's fullest and became one of the most effective characters in the party.

It personally makes me think of night crawler from X-Men his tail is prehensile and he is demon like minus the horns

 I've played a Tiefling with a prehensile tail my DM and I had an understanding where the tail could pick up a dagger though the tail was too weak to wield a dagger, He could use his tail to hold a glass or open a door etcetera. 

Real fun character to play he'd  play the violin by holding the bow with he's tail 

Quote from Kuven >>

 I've played a Tiefling with a prehensile tail my DM and I had an understanding where the tail could pick up a dagger though the tail was too weak to wield a dagger, He could use his tail to hold a glass or open a door etcetera. 

Real fun character to play he'd  play the violin by holding the bow with he's tail 

 
Interesting.  If they could pick up a dagger with their tail, how was the tail too weak to use the dagger?  

I am not saying your character was wrong, it was your character so your choice, I’m just curious about how they could pick up a dagger but could not use it?  

As others have said about balancing is great, and as for it being appropriate conceptually, before 4e made all tieflings just be from a deal with Asmodeus, the original tieflings had an incredible wide variety of appearances. It's kind of sad they became so standardized around one specific appearance. So, as long as it's balanced mechanically, go for it! It's quite appropriate for the concept of tieflings!

In the old Planewalker's Handbook there was even a full chart of traits for tieflings that goes far beyond the 4e & 5e limited appearance. That book (and a lot of the Planescape stuff on DMs Guild) is a great resource with a very high percentage of lore rather than mechanics that work just as well for 5e.

Quote from TWForgeCleric >>

Quote from Kuven >>

 I've played a Tiefling with a prehensile tail my DM and I had an understanding where the tail could pick up a dagger though the tail was too weak to wield a dagger, He could use his tail to hold a glass or open a door etcetera. 

Real fun character to play he'd  play the violin by holding the bow with he's tail 

 
Interesting.  If they could pick up a dagger with their tail, how was the tail too weak to use the dagger?  

I am not saying your character was wrong, it was your character so your choice, I’m just curious about how they could pick up a dagger but could not use it?  

 Probably written the same way all prehensile weak limbs are written - i.e. Loxodon trunks and Simic Hybrid tentacles.  Generally speaking, the idea is that they're strong but clumsy.

Trunk. You can grasp things with your trunk, and you can use it as a snorkel. It has a reach of 5 feet, and it can lift a number of pounds equal to five times your Strength score. You can use it to do the following simple tasks: lift, drop, hold, push, or pull an object or a creature; open or close a door or a container; grapple someone; or make an unarmed strike. Your DM might allow other simple tasks to be added to that list of options. Your trunk can't wield weapons or shields or do anything that requires manual precision, such as using tools or magic items or performing the somatic components of a spell.

Grappling Appendage. You have two special appendages growing alongside your arms. Choose whether they're both claws or tentacles. As an action, you can use one of them to try to grapple a creature. Each one is also a natural weapon, which you can use to make an unarmed strike. If you hit with it, the target takes bludgeoning damage equal to 1d6 + your Strength modifier, instead of the bludgeoning damage normal for an unarmed strike. Immediately after hitting, you can try to grapple the target as a bonus action. These appendages can't precisely manipulate anything and can't wield weapons, magic items, or other specialized equipment.

In order to be functionally prehensile, a tail needs to have a lot of short vertebra in it.  It can be quite strong when it comes to gripping, but this puts a very large limit on how much force it can exert.  It's pretty hard to simultaneously get a tail that can swing hard enough to effectively attack while simultaneously be flexible enough to grasp objects.

Quote from RodneyMcNeely >>

Honestly, I don't see why you can't roleplay the heck out of this.  You don't even need to homebrew anything.

Say you're running a Two-Weapon Fighting build.  Just pretend you've got the rapier in your right hand, and maybe you keep switching the main-gauche from your left hand to your tail.

You really can describe it any way you like without needing to jimmy the rules.

I think this is the best way to do it, flavor play, not complicating the character's action economy.  Don't even give it reach.  It would just be a fun way for a two weapon combatant to explain how they deliver one attack to the enemy in front of them and one to the one behind them.  

Actually play this out on a character that's currently on hiatus. The mechanic is beast barb for the tail parry. However we just hand waive the tail growing and say its just there. It doesn't really do anything crazy weird or broken. Wildest case scenario thus far was using it to help stake a tent, or get yanked drag-bar style out of combat, but both of those could be done with just about anything. In combat it's doing its barb thing and there's no real good way to utilize prehensile mechanics without additional action economy anyway

I mean sure, the argument has and usually is made that it could offhand a weapon but as was already mentioned here its kinda bad for that anyway. Closest other mechanical thing i can think of comes from Rogue/Thief. Maybe I'm not creative enough with it.

Quote from xMakeRx >>

Actually play this out on a character that's currently on hiatus. The mechanic is beast barb for the tail parry. However we just hand waive the tail growing and say its just there. It doesn't really do anything crazy weird or broken. Wildest case scenario thus far was using it to help stake a tent, or get yanked drag-bar style out of combat, but both of those could be done with just about anything. In combat it's doing its barb thing and there's no real good way to utilize prehensile mechanics without additional action economy anyway

I mean sure, the argument has and usually is made that it could offhand a weapon but as was already mentioned here its kinda bad for that anyway. Closest other mechanical thing i can think of comes from Rogue/Thief. Maybe I'm not creative enough with it.

Attach a bladed implement to that tail, and it becomes a reasonably deadly weapon.  One that could be used to fend off attacks from behind or brought forward and used to stab an enemy in the face while grappling.  

There are many ways you could weaponise your tail, and it surprises me that in a world where tails are pretty standard, nobody has found a way to utilise their combat potential.  

Quote from TWForgeCleric >>

Quote from xMakeRx >>

Actually play this out on a character that's currently on hiatus. The mechanic is beast barb for the tail parry. However we just hand waive the tail growing and say its just there. It doesn't really do anything crazy weird or broken. Wildest case scenario thus far was using it to help stake a tent, or get yanked drag-bar style out of combat, but both of those could be done with just about anything. In combat it's doing its barb thing and there's no real good way to utilize prehensile mechanics without additional action economy anyway

I mean sure, the argument has and usually is made that it could offhand a weapon but as was already mentioned here its kinda bad for that anyway. Closest other mechanical thing i can think of comes from Rogue/Thief. Maybe I'm not creative enough with it.

 

Attach a bladed implement to that tail, and it becomes a reasonably deadly weapon.  One that could be used to fend off attacks from behind or brought forward and used to stab an enemy in the face while grappling.  

There are many ways you could weaponise your tail, and it surprises me that in a world where tails are pretty standard, nobody has found a way to utilise their combat potential.  

 That should tell you that the tails probably can't be weaponized, then. As has been mentioned, the best way is probably to just roleplay it without looking to gain extra advantages.

Strapping sharp object to your limbs is usually not a good idea. There's a reason why you don't see people running around with knives strapped to their knees and so on. And just because you have a limb that is strong enough to lift something doesn't mean that limb is strong enough to use the same object as a weapon. I can lift a a machine gun in one hand but I can't use the very same hand to fire properly. Not to mention that just because you have a tail doesn't mean you automatically can see what's happening behind you.

Quote from TWForgeCleric >>

Quote from xMakeRx >>

Actually play this out on a character that's currently on hiatus. The mechanic is beast barb for the tail parry. However we just hand waive the tail growing and say its just there. It doesn't really do anything crazy weird or broken. Wildest case scenario thus far was using it to help stake a tent, or get yanked drag-bar style out of combat, but both of those could be done with just about anything. In combat it's doing its barb thing and there's no real good way to utilize prehensile mechanics without additional action economy anyway

I mean sure, the argument has and usually is made that it could offhand a weapon but as was already mentioned here its kinda bad for that anyway. Closest other mechanical thing i can think of comes from Rogue/Thief. Maybe I'm not creative enough with it.

 

Attach a bladed implement to that tail, and it becomes a reasonably deadly weapon.  One that could be used to fend off attacks from behind or brought forward and used to stab an enemy in the face while grappling.  

There are many ways you could weaponise your tail, and it surprises me that in a world where tails are pretty standard, nobody has found a way to utilise their combat potential.  

 You can tie a razor blade to a ponytail but it wouldn't be an effective weapon.  Adding a pointy bit isn't enough to make an effective weapon, you've still got to have the strength and leverage to apply that weapon effectively.  See my previous post for the problem with attacking with a highly flexible, prehensile tail.

Since we're talking fantasy anatomy it may be a moot point, but I can't think of an animal with a truly prehensile tail that can also use that tail as a weapon.  Only tail weapons I've seen in present (and I think prehistoric) creatures are reptiles and dinosaurs who use their tails basically as blunt clubs, but I can't see extrapolating finesse out of that usage.  Everything else tails are for stability/balance with some having an arguable communicative function.

I suppose the pseduopod of the plasmoid UA could give some rules function precendent if you halve the range, but I really think Tieflings don't need another feature laden onto them so it's probably best, if you really wanted a manipulative tail, to just flavor it as primary or off hand use narrationally with no real racial feature mechanic.

My problem is that a Tieflings tail doesn't serve any purpose, and at that point, why even bother having a tail?  It feels like there's no reason for them to have a tail if their tails are useless.  

They might as well be a human with fire resistance, which you could probably yet on a variant human via a feat anyway.  So at that point, why not just take a variant human, give them fire resistance via a feat and roleplay them as having cursed blood or something due to their ancestor's interactions with devils.

Or V.Human with fire resistance, who is a warlock of the fiend if you wanted specific devil related powers.  

What is a USP of a Tiefling at that point?