Does Tiamat rule over lesser devils?

I am in a situation where the Cult of the Dragon is in direct conversation with a bearded devil that wishes to wreak havoc on their camp. (It’s a long story that I’m happy to provide details with if asked)

However, my question is this: Although Tiamat is no longer an archdevil and only resides within Avernus of the Nine Hells, does she have any control in this situation to tell the bearded devil to step off? I have my own thoughts, but I’m curious to start a discussion on it.

What is the in-universe explanation for why succubi, who were demons, became devils, then became “neutral evil fiends”?

The Forgotten Realms wiki page on succubi tell us (specifically in footnote 1) that in 1e, 2e and 3.Xe1, succubi were chaotic evil demons, but then were retconned to be lawful evil2 devils in 4e and have now just been made into generic neutral evil "fiends" in 5e, presumably in an attempt to avoid contradicting any previous editions’ lore.

Unlike with the Shadar-kai, I believe there was supposed to be some kind of canonical in-universe lore reason as to why these demons became devils. What was that reason? I assume it appears in some 4e material somewhere? I’m only really familiar with 5e material…

Furthermore, does 5e give any sort of in-universe lore explanation as to why they are now neither devil nor demon? The 5e Monster Manual entry doesn’t really explain that besides briefly mentioning that they "can be found in service to devils, demons, night hags, rakshasas and yugoloths", again presumably to avoid contradicting any previous editions’ lore, but without explaining why this is now the case.

1 Actually, the footnote on the Forgotten Realms wiki page only says 3e, but I know it was still true in 3.5e because of Neverwinter Nights 2, which was a video game based on 3.5e. In this game they were considered demons, which is incidentally my introduction to D&D and why I consider succubi being demons to be what they "should" be.

2 I say "lawful evil", because that’s what a devil’s alignment is, but I’m aware that 4e changed the alignment system, so it might not be so accurate to claim they were "lawful evil" in 4e, but at the very least, in the context of D&D overall, they would have been considered lawful evil all the time they were considered to be devils.

Does Devil’s Sight enable one to see into Hunger of Hadar?

The hunger of Hadar spell (PHB, p. 251) creates a black void of darkness, which cannot be penetrated by light. This means that no one can see in, and those inside cannot see at all, which is a handy way to damage and control enemies since they don’t know which way is out.

This also means that characters cannot easily attack those inside.

The Devil’s Sight eldritch invocation, however, enables a warlock to see normally in magical and non-magical darkness.

Does this enable the warlock to see into the area of blackness created by hunger of Hadar and attack creatures inside? Are there any other sight mechanics that allow someone to see in or out of the spell?

does Voice of the Chain Master effectively replace Devil’s Sight If the warlock’s familiar is an imp? [duplicate]

to me, the Voice of the Chain Master invocation reads that it completely upgrades the basic Find Familiar spell, increasing the range of the telepathic communication to 100 feet and no longer requiring an action to perceive through its senses.

I have read Crawford’s Sage Advice saying you still require an action, but that was 4 years ago and he is known to change his rulings (though it might not have changed, just that people have not asked in a while)

What happens when a player violates two devils’ contracts?

I’m currently preparing a session in my campaign where one of my players is going to consider signing a contract with a devil. While writing the contract, I noticed something that seems like a loophole that definitely isn’t worth it, but it looked interesting nevertheless. Imagine the following scenario:

  1. A PC signs a contract with two different devils;
  2. Both contracts state that the respective devils get the PC’s soul upon death, may the PC violate the contract.
  3. The PC goes and violates both contracts.

What would happen here? Would the second devil never take the contract? Does the second contract become invalid? Does the second devil have to estimate whether the PC doesn’t violate the first contract, and take the contract based on their guess?

Additionally, if the PC breaks violates either contract, would there still be a reason to follow the second contract?

Does Mordenkainen’s Private Sanctum prevent Devil’s Sight from working?

I could compare it to this question about whether the Devil’s Sight invocation allows one to see into Hunger of Hadar, but that question has an easy answer, due to the ‘blinded’ condition it affects the person inside with. However, the spell Mordenkainen’s Private Sanctum does not give the ‘blinded’ effect- it merely states that, in one of its properties:

The barrier of the warded area appears dark and foggy, preventing vision (including darkvision) through it.

How should a Warlock’s Devil’s Sight invocation work?

The Warlock Invocation Devil’s Sight (PHB p108) is described like this:

You can see normally in darkness, both magical and nonmagical, to a distance of 120 feet.

How should this be interpreted given the undefined use of the word “normally” and the description of a real devil’s Devil’s Sight ability?

Barbed Devil (MM p.70):

Magical darkness doesn’t impede the devil’s darkvision

Do devils have to speak their true name backwards?

In reading Pools of Darkness

I have not seen any evidence of this in other D&D books or actual D&D rules.

So, is this just Pools of Darkness or is it an official rule in any editions of D&D because it seems like it would be unbalancing for any idiot with a spellbook who hears you talk for two sentences to exercise control over you.

Are any editions of D&D in which this is a rule?

Does an Open Hand monk’s Wholeness of Body close a Horned Devil’s infernal wound?

Basically the exact same question as this one, but for 5e (since the other question is about Pathfinder).

A Horned Devil can inflict an “infernal wound” with its tail attack , which essentially causes bleed damage over time (bold only emphasis mine):

Tail. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 10 (1d8 + 6) piercing damage. If the target is a creature other than an undead or a construct, it must succeed on a DC 17 Constitution saving throw or lose 10 (3d6) hit points at the start of each of its turns due to an infernal wound. Each time the devil hits the wounded target with this attack, the damage dealt by the wound increases by 10 (3d6). Any creature can take an action to stanch the wound with a successful DC 12 Wisdom (Medicine) check. The wound also closes if the target receives magical healing.

A monk with the Way of the Open Hand monastic tradition has the Wholeness of Body feature:

Wholeness of Body

At 6th level, you gain the ability to heal yourself. As an action, you can regain hit points equal to three times your monk level. You must finish a long rest before you can use this feature again.

which doesn’t explicitly say that it’s magical healing, but the implication is that you can do this because of your Ki, which is described as magical under the “The Magic of Ki” section of the monk class (bold emphasis mine):

Monks make careful study of a magical energy that most monastic traditions call ki. This energy is an element of the magic that suffuses the multiverse—specifically, the element that flows through living bodies. Monks harness this power within themselves to create magical effects and exceed their bodies’ physical capabilities …

So, is a Way of the Open Hand monk’s Wholeness of Body ability considered to be magical healing such that it can be used to close an infernal wound?