Do tiefling and half-fiends go to hell/abyss when they die? Do Asimaar and Half-angels go to Heaven?

Do tiefling and half-fiends go to hell/abyss when they die? Do Asimaar and Half-angels go to Heaven?

Depends on setting.

Do humans?

Depends on setting.
But default D&D cosmology says if you're not a humanoid type it's doubtful you even have a soul, and if you're celestial type or fiend type you definitely don't have a soul.

Yes.

If they never dedicated themselves to a deity.

However, deities, while not in need of worshippers certainly do always benefit from souls in their courts, and such plane touched scions are often major targets of different faiths hoping to recruit them early.

Often deities relating to a corresponding native plane don't bother trying too hard besides sabotaging the competition.

But in the end, you get clerics of Pelor and the like keeping fingers on the pulse of half-fiend and tiefling communities to try to seek out dedicated souls and bring them into the flock. Given Tieflings and Half-fiends are usually societal outcasts, the hospitality of these churches will end up as indoctrinating as any cult and you end up with lots of paladins with fiendish ancestry, zealous and dedicated staunchly to their new deity.

Good Asimaar go to Heaven, evil Tieflings go to Hell, finicky cases with argumentative petitions go to Wall.

Tieflings go to hell unless there is an intercession from another party, like a god. That's ok though because tieflings get to hang out with their parent and their side of the family, even come Into full demonhood.

Aasimar, according to 5e have some kind of personal guardian angel, that's like a guarantor for your soul.

Would you expect the devotees of Asmodeus to have more difficulty bringing Asimaar into the fold?

Depends on the aasimar and tiefling in question. I'm not aware of any canon statement that says that aasimar get a free pass, nor that tieflings are automatically damned. Like humans, it depends on how they acted and what they did in life.

This . The only creatures this doesn't apply to are outsiders - full-blooded celestials and fiends are pretty much made of the essence of the relevant plane. They also don't have free will.
Elementals are made of elemental stuff (surprise!) and just dissolve back into the surroundings when killed.

Everyone else - half-fiend/celestial, aasimar, tiefling and so on - gets judged by their actions. They may have a predilection for a certain alignment, but they still have free will.

In FR specifically they go to the Wall (or take the offers of hell) if they haven't chosen a deity before their death, same as everyone else.

This is all D&D of course.

They are flesh and blood, and born rather than formed. So, in death they are treated like all other mortals. Maybe.

>and if you're celestial type or fiend type you definitely don't have a soul.
because you're already a soul?

so what is stopping a wizard from perma-poly morphing a full blooded celestial/feindish creature into an asimar/teifling? all youre basically doing is granting them human blood and free will right?

Only in terms of accessibility- aasimar probably don't cluster in ghettos.

Afterlives are alignment based, it depends upon their actions and what they're mechanically labelled as.

IE the only reasons Alignment should matter anymore.

Angels are also less lewd than demons.

In my setting, reincarnation of the soul is a thing, with the soul split into two basic chunks - memory or identity, and life force. Life force is always split off from the other two to reconstitute the begins of a new soul, and the identity is sent to the most fitting afterlife for the person's faith and deeds.

Usually morality is simply part of identity, and does not constitute its own chunk, but divine or infernal heritage gives aasimar and tieflings inherent leanings towards good or evil respectively, a sort of supernatural essence that is seperate from the average mortal soul. And that must always return to the sphere of the devil or angel that was responsible for the creation of the special person, no matter where their identity goes. So a part of their essence will always go to hell or heaven, yes.

Tieflings and Aasimar are mortals, just planetouched. They have a tendency towards the lower and higher planes, but nothing is guaranteed. Evil Aasimar and Good Tieflings will go the other way.
Half-Fiends and Half-Celestials are another story. They're so filled with the power of Evil and Good that they don't really have the option for choice, just like their extraplanar parent. Their children will be given the option of choice, but they will not.

I don't think there's anything canonical that applies exclusively to them, but I'd say it depends on their alignment/religion like any other creature

>But default D&D cosmology says if you're not a humanoid type it's doubtful you even have a soul
Source on that?
Not doubting it but I thought only outsiders/elemental (and maybe constructs) lacked a soul due to lacking a body/soul duality, I've just assumed anything with an int higher than 3 had one (By RAW a sapient being can't naturally have it lower than three)

Well by RAW, duration of permanent is still a spell duration, just one that runs out in an indefinite point in time in the future (IE never outside of interference) so you could argue they're still a celestial/fiend just under a polymorph spell

Now if you want to truly permanently change one you need a polymorph with an instantaneous duration as that would mean they're now a asimar/teifling not an angel/demon transformed into one

No.

>polymorph with instant duration

how does that even work? isnt permanence supposed to change a spell from a de-buff to a permanent part of their character chart?

why wouldnt the permanence spell make the TRUE polymorph spell part of them forever and not be disspellable?

if anyone wanted to change them back wouldnt they also need another perma-poly spell, wish, or divine magic to re-change their base race?

>how does that even work? isnt permanence supposed to change a spell from a de-buff to a permanent part of their character chart?
>why wouldnt the permanence spell make the TRUE polymorph spell part of them forever and not be disspellable?
If you look at the rules "Permanent" means the spell is ongoing, affecting the target until it gets dispelled or dismissed. In the case of a polymorph, the target is under the effects of a spell until it's over (ideally never), but still under a spell's effect. Even by slapping a permanency spell on another effect you're just changing the spell's duration to permanent, which does nothing to prevent it from being dispelled or otherwise dismissed if possible.
Instantaneous on the other hand means the moment you cast it the spell is over, it's done it's thing, whatever state the target is in afterwards cannot be affected by dispells or the like because that's how the target is now, not because something is affecting the target.

For example, you use a polymorph spell with permanent duration and the target walks into an antimagic field they change back because the spell currently affecting them is suppressed in the field and will affect them again the moment they walk out because it's duration is still ongoing.
Use a spell with instantaneous duration and they don't change back because there is no magic affecting them so the antimagic field does nothing

>if anyone wanted to change them back wouldnt they also need another perma-poly spell, wish, or divine magic to re-change their base race?
You could slap two polymorph spells on and effectively reverse the effects of the first yes, but dispelling it would be easier. It's not reverting the change in that case, just two ongoing effects that cancel each other out

But yeah basic gist is casting a permanent polymorph into teifling on a demon = demon in the shape of a teifling because a spell is trapping him in the shape, instantaneous polymorph = demon is now a teifling

Then what's the best way to metamagic a poly spell to be instant? Or a more perma wayto change them besides permapoly?

The thing is, there isn't one unless you want to get into epic level spells or homebrewn effects
Which has always sorta annoyed me, at the very least I've always thought there should be a way to make minor effects like alter self instantaneous much less larger changes like polymorph, but sadly, unless there's some obscure book I don't know about as far as I'm aware in 3.5/PF it's not possible by canon 1-9 level spells