Tiefling Tail Posture
The consistently amazing BG3 tiefling fanart inspired me to draw out how my characters' tails would emote very soon after I started playing, and I finally went through and cleaned up those sketches today. I based these primarily off of cat behavior with some liberties to account for how they look and behave in-game. I interpreted the triangular ridges on the base of the tail as spines that can raise in extremes, because it's more fun. As always, thanks to the artists at Larian for going unreasonably hard with all the bumps and ridges in unmentionable places.
These are what I picture for my Dark Urge specifically (a lot of them don’t apply to how I imagine Karlach, for example) but feel free to draw from them for any tiefling characters.
If you can't read my handwriting (sorry), I transcribed it in the ALT text. More rambling about how these work for Bran under the cut.
Happy, Contented
Before Orin attacked them, this was very rare for Bran, and it is still generally fleeting. His tail will curl upward as shown when he laughs unexpectedly, but when he feels happy for longer periods of time, the curl is usually somewhat hard to notice unless you know exactly what you're looking for. It'll curl up tighter when he is touching someone he feels close to (whether platonically, romantically, or familially), but this only became truly pronounced after his urges were gone. Before that, his tail posture was often more angry or anxious looking when touching another person, especially for prolonged periods of time, and it would tick and lash frequently out of its curl before returning to it.
Agitated, Angry
This one has a lot of nuances, because pre-Orin Bran spent a lot of time being openly angry and irritated to a degree. Anger is sometimes a problem for post-Orin Bran, but he is much more inclined to repress it to avoid upsetting or scaring the party, so it is far less visible especially when it's directed at them. It usually manifests with a slightly raised base and a tick to one side or the other in response to a particularly strong or inconvenient urge, and it'd happen very frequently whenever someone came too close to them or touched them unexpectedly. This didn't go away when he got rid of his urges, but after a few years it got to the point that it was no longer as stark. This is a tell they can suppress with a lot of effort, but they aren't reliably successful, especially post-Orin.
Lashing was something he did when he was too angry to care about hiding it (even though he often could have), which happened with people he was comfortable around, primarily Gortash and Orin. Post-Orin, this only happens when they get so mad they can't suppress it. He may also lash and tick his tail back and forth on purpose when trying to be intimidating because it tends to trigger a conscious feeling of being hunted in most people, especially while he is also threatening them verbally and via Thamaturgy/Druidcraft.
Fear, Shock
This is very rare for Bran; he isn't prone to panic or expressing extreme fear in anything but the most extreme circumstances. When he is that afraid, it typically shifts to agitation or anxiety fairly quickly, especially pre-Orin. It'll happen briefly in moments when they're surprised by an ambush, but they won't hold this posture for longer than a second unless they're in the middle of a flashback. They almost never experience the level of disgust that'd trigger this reaction (I literally can't think of anything that would cause that for them. Desensitization is a trip).
Coy, Aroused
This one is also nuanced because way it usually manifests is not something that's natural for Bran. Pre-Orin, they'd typically curl and flick their tail deliberately to accompany flirting, but it also tended to intimidate people with how overt it is and how it often also triggers the "being hunted" feeling. In his relationship with Gortash, both of parts of that were intentional. The motion looks very intentional too: it moves to one side and then pauses a little on the way back up rather than continuously flicking to the other side, and the whole arc is slow enough that this is immediately noticeable to anyone paying attention. Bran keeps the habit post-Orin, at which point it becomes more or less involuntary while flirting and sometimes backfires because of the intimidating element. It does get a little subtler eventually, tending to only flick to one side or back and forth a couple times before he stops it and leaves it curled.
The raised base only happens when Bran receives unexpected and welcome advances, and it is typically quite brief. The spines and base will both raise during actual foreplay or sex, but not typically outside of that. The spines raising generally doesn't last for much longer than a moment for most of that time unless something is going either very right or very wrong.
Relaxed, Calm
This is basically just the game's default/walking posture, so there's not much to add. It'll sometimes sway gently when Bran is standing still or thinking (though it's not like that when they're thinking deeply, especially not post-Orin). Bran's boredom would usually look like subtle swaying when he isn't doing the usual Tav stretch, and it would be much subtler than Karlach's style of fidgeting.
Thoughtful
The arc Bran's tail flicks along when thinking deeply is similar to the one it follows while flirting in terms of how their tail turns over, but it is more elliptical and continuous than the way it flicks when they're being coy. It is also something they do naturally/on accident as opposed to doing it on purpose, which more perceptive people are generally able to tell. Pre-Orin they usually suppress it to avoid playing their hand unless they're around Gortash, Orin, or other people they're comfortable emoting openly around, or when they're trying to scare someone deliberately.
Post-Orin they aren't usually aware enough of it to suppress it until it's already started happening, and they don't feel compelled to do that around the rest of the party. It rarely makes the others uncomfortable unless he starts doing it while looking at and talking to them (he also tends to trail off and look through them when very deep in thought, which can be very disconcerting). None of them are willing to admit it, so Bran tends to let it happen because he's too preoccupied to notice their discomfort for himself. That unease largely went away when they realized that it wasn't intentional (at least for everyone but Astarion, because he took a while to believe it).
Tired, Injured
This is a behavior that they tend to hide until they no longer can, both pre- and post-Orin, so it is a very obvious cause for concern for the party. Dipping as low as shown and dragging usually happen when their headaches and brain fog/lethargy get bad, or when they've lost quite a bit of blood. It'll start to dip subtly out of their normal posture after a long day or if they stay up past the time they'd usually fall asleep, but it doesn't happen to the degree shown in the sketch for regular tiredness. It won't bump or drag against the ground unless something is well and truly wrong.
Curling his tail around himself is a self-soothing behavior, so it happens in response to pain at a level he cannot ignore (and he can ignore a lot). This is for the very bad and sharply painful headaches as well as grievous injuries. Dragging and curling his tail for long periods are things the party eventually learns they need to force him to stop for whenever he can't suppress them in time.
Anxiety, Discomfort
This is something he suppresses whenever possible, but he's only good at doing it pre-Orin. Before he was attacked, Bran never let this happen unless he was too overwhelmed not to, and he would usually convert the motion into agitation after a few seconds. Post-Orin, he'll still suppress it, but he'll often get anxious and disassociate enough that he won't realize it's happening, particularly in crowds and indoors in loud or echoing spaces full of people. If they aren't completely out of it, it will usually manifest as a brief curl or sharp flinch into this posture before it smooths out into something that's more subtle, but the tip of their tail will still generally stay curled around them until they can distract themself or get away from whatever triggered it.
This is also something that happens post-Orin when Bran feels guilt, which they were previously more or less numb to. Except with regards to specific people they cared for enough to grieve, they were resigned to their role and their urges, so the remorse they felt for killing was minimal and much easier to repress or convert to a more manageable emotion. Other than his worst bouts of self-loathing, I can't think of much of anything that'd trigger this reaction for disgust.
