So what would it mean for an Elf to attain the kind of soul that a human has? Such an event is attested in old tales, chief among them the tale of Undine, though arguably the Tale of Beren and Luthien in which the Elf-woman Luthien Tinuviel is granted the Doom of death to join her human love Beren in death also entails an Elf attaining a soul. There is also the song of the Cup of Lethe and the White Sword, which involves a half-Elfin maid giving up her Elfin nature to join human society (yeah, yeah, shameless on my part, I wrote that epic).
|Alas for Undine, she is actually rejected by Huldbrand|
and so is doomed to return to the waters as mere seafoam ...
In my current Greyhame game, the Elfin character Blackleaf is engaged in a quest to attain a soul for herself, in which she generally ineffectually attempts to win the true love of local blacksmith Aldir. The lore is that an Elf would gain a soul if married to a Good human, because of the union of flesh and all that which marriage entails; and so Blackleaf, after having a drunken fling with Aldir, encouraged the paladin Aethelwulf to convert Aldir to Goodness. But alas for Blackleaf, when Aldir listened to Aethelwulf, he was so impressed with the tenets of Goodness that he abandoned his illicit relationship with the Elf-woman and devoted himself entirely to the Good cult and its tenets of sinless living! Now Blackleaf hangs around and tries to impress him, but he generally turns his nose up at her because of poor reactions rolls.
Still, perhaps Blackleaf could someday win back Aldir's affections and attain a soul through marriage to him. Or through some other means, like a wish spell, or some other miracle.
So what would happen then?
My current inclination would be to adapt the rules for dual-classing from 1e AD&D. An Elf, e.g. Blackleaf, who attained an immortal soul would forever turn their back on the "class" of Elf. No more could she advance as a kind of fighter/magic-user, and no more would she be able to cast spells and fight while wearing armor. Instead, an Elf with a soul would have to choose a new class--fighter, thief, magic-user, or cleric (or paladin, ranger, or illusionist), and advance as such for the rest of her days. (Assassin and Druid would be denied as class options here, because an Elf with a soul must be Good, as the soul that was attained was through the union in flesh to a Good human ... I suppose a soul gained another way, i.e. through wish might align differently).
Per dual-classing, the Elf would start over at first level of experience in her new class, though retaining her hit points as already accrued, and taking the better saves and THAC0 between the new class and her current class. Otherwise, she must act within the abilities of her new class, or forfeit any experience gained during the session/expedition, i.e. an Elf that dual-classes to magic-user that nevertheless takes to wearing armor during an adventure would receive no experience at the end of that adventure.
As class options, fighter and magic-user would both be immediately available for the former-Elf to dual class into, as those are the classes most similar to an Elf's proclivities anyway. As a fighter, she would not be able to cast spells until attaining a level in fighter equal to her level in the Elf class; as a magic-user, she would not be able to wear any armor or wield any weapons other than a dagger, but could still cast all of the spells known to her from her levels as an Elf.
The other classes would be available, but would require a human sponsor who could train the Elf-cum-human in the particulars of her new-chosen class. Honestly, this shouldn't be much of a roadblock to selection; either the character should know another PC of the chosen class who can sponsor them, or it involves the introduction of a new NPC who can do the same. At the most, this might involve a session's worth of gaming/role playing to find an NPC who could sponsor the Elf in their chosen class.
So that's my initial thoughts on what it means for an Elf to gain a soul in my game. I can already see that I need to relax the alignment thing I mentioned above--just because an Elf married a Good human and received a soul doesn't mean that she doesn't immediately use that soul to turn to Evil. The whole point of a soul is to be able to choose!
Still, this will be my basis as things continue. Perhaps Blackleaf will be successful in her machinations with Aldir. Ultimately, I expect she'll find a means to attain a soul one way or the other, regardless, and we'll move forward from there!