#the hells

The "Blood War" that Karlach and a few other characters mention in Avernus? That is devils vs. demons. In fact, devils were originally celestials themselves, that became corrupted in their efforts to stop the tide of demons. The remaining celestials turn a blind eye to the Hells (or actively encourage its existence) because the devils are the forces holding back the encroaching Abyss, a far greater threat than they themselves could ever be.

The major area that we see this hierarchy expressed is in the layers of the Hells, the different dominions. Zariel is the least powerful Archdevil, ruling (and fairly recently come to power, too) the First layer: Avernus. The second-strongest Archdevil, of the Eighth layer, is Mephistopheles, superseded only by Asmodeus, the Archdevil of the Ninth layer.

With regard to the overall power level of the archdevils, this is where the exact answer gets a bit fuzzy. It has varied by edition, and personal take on the lore, but the ultimate point of agreement is this: no matter how powerful you personally think the Archdevils may be, Asmodeus is the exception. He has at least the strength of a greater god (one with a wide and expansive domain crossing multiple worlds), not even including the additional power he would be able to bring to bear by virtue of his domain, were a conflict to take place within it.

Raphael and Mizora, the two "devils" we have the most direct experience with as players, are not true devils at all, but rather cambions: half-devils. This allows them to skip some of the steps of power development usually required for devils to increase their power, but also stratifies them a bit with that development. Their primary advantage is freedom of movement between the planes, granted to them by their mortal parentage.

(The above is just part of the reason why Raphael's "plan" is so laughably doomed to abject failure, but that is a meta for another day.)

On to souls: why do devils need them, how do they get them, and what do they use them for?

Devils have an entire society based on backstabbing, conniving, one-upmanship, and, most importantly: paperwork. The devils - especially the Archdevils - are always looking for ways to get one up on each other. Not even necessarily to take their positions (although if it so happens to work out that way, all the better), but just to have information, leverage, even one point of superiority over them, etc.

One of the primary ways devils can bolster their power levels is based on the number (and quality) of souls under their command. Those souls, when willingly signed away, do not go to the usual Fugue Plane upon death, to be taken by whatever god values them most highly to their own afterlife. Instead, they go directly to the Hells. There they begin the standard progression of lowest-ranked devils: tortured to wring out what magical energy can be gleaned from them, then starting as lemures on the lowest rungs of the hierarchy, contributing to the infernal economy and adding to their ruling Archdevil's power, ranks, and influence.

Unwilling souls can be used, but they are generally considered to be the lowest quality, used mostly for things like soul coins, etc.

Note: this is where this is going from primarily lore-backed meta to meta I am extrapolating onto a bit, based on what seems logical to me.

If souls can only be willingly signed away, why can Cazador pay for his contract with Mephistopheles by trading the souls of others? Based on the legalistic evil of devils, my take is that, by the properties of magic and vampirism in the Forgotten Realms, "legally" speaking, the spawn he creates are enough a part of him that he is able to speak for their souls in their stead.

Based on this property, and the aforementioned power scale of the devils, the thought arises: does it really make that much sense for the Dark Urge to just be able to waltz into the Eighth layer of the Hells and steal the Crown of Karsus? It would make much more sense if it was willingly given - or at the least, allowed to be taken - by Mephisto, who stood to gain something much better than a bauble of not much use to archdevils: souls, and a multitude of them.

If Cazador is able to sign away the souls of his spawn, due to them being magically recognized as a part of him, I believe the same would be true for mindflayers turned by the controlled Netherbrain. The goal of the Dead Three was to ultimately transform an untold number of denizens of the Forgotten Realms, to take their souls and the power from their worship away from the gods (they never claimed to be brilliant strategists, we'll leave it at that). Mephisto, then, would stand to gain a great deal in being able to make use of those souls himself.

I do think that rather than a heist, Durge and/or Gortash made a deal with Mephisto for the Crown in exchange for the souls of any transformed mindflayers. A win-win for both parties, by all accounts.