According to the Dungeonmaster's Guide (page 24):
When a creature dies, its soul leaves its body, leaves the material plane, travels through the astral plane, and remains on the plane where the creature's deity resides. If the creature did not worship a deity, its soul goes to the plane corresponding to its alignment.
Furthermore, it is also possible for a soul to be claimed by a powerful demon. For example, the definition of the Hades states (page 63):
Hades is the destiny of many souls who are not claimed by the gods of the higher planes or the devilish rulers of the lower planes.
This statement is supplemented by a series of references to the archdevils making deals with mortals, in the section on the Nine Hells. For example, about Dispater we read (page 64):
He is more obsessed than most demons with surprising deals with mortals in exchange for their souls …
Based on this we can conclude:
- Souls that worship deities whose kingdoms are in the Nine Hells
- Souls claimed by the archdevils due to some treatment
- Legal evil souls who are not claimed by any deity or demon.
Also, a specific unofficial response from Ed Greenwood's Forgotten-Realms can be found in a series of tweets on March 1, 2020, answering the question of how Faerûnian souls (not collected by deities) could end up in the Nine Hells, aside from Faustian's bargains.
Toril's mortal souls end up in the Nine Hells due to curses, the bargains you mention (sometimes for lost bets), and sometimes for other deadly actions taken in life, such as the magical rituals that unintentionally bind those who lead to Perform the ritual. to a particular archdevil, or other traps set by demons to catch the unwary. Even sloppy summons can allow a demon to reclaim the summoner's soul in future pay, or a poorly cast spell can make a trap possible. The forms are surprisingly many, so the supply of souls to the Nine Hells is fairly constant, if not widely noted in tradition.