In the first-edition DMG, magic-users choose three starting spells by rolling on “offensive spells”, “defensive spells”, and “misc. spells” tables. These tables cover every first-level spell – except two. Says the DMG: “Note that both Nystul’s Magic Aura and Tenser’s Floating Disc must be located by the character; they can never be known at the start.”
Looking at these excluded spells, two possibilities spring to my mind. The first and almost certainly the right one is that these two spells happen to be the least generally useful of the first-level spells and most likely to hamper a fledgling wizard, so they’re left out. The second is that there’s a generalizable rule at work: “proper name spells” can only be obtained as treasure. After all, the two excluded spells are the only two first-level spells with a wizard’s name attached.
A rule about spells available at character creation is minimally interesting. But in 1e as in later editions, the wizard gets to pick new spells at every character level. What if we excluded Mordenkainen’s, Bigby’s, and any other Somebody’s spells from this auto-learn list? We’d add a rule like this: Any spell that starts with a proper name and an apostrophe can’t be learned at character creation or as part of leveling up. It can only be found as treasure.
It would make some game-world sense. Spells with still-intact author attribution might be relatively new spells, jealously guarded by the original creators or their pupils. These spells would act as a sort of trademark. A giant clenched hand appears in the sky? Bigby (or your campaign’s equivalent) must be in town.
Both the 1e and 5e spell list include about 1 or 2 named spells per spell level, making them occur in about the right frequency for “rare spells”. Furthermore, the named spells tend to be somewhat odd. Many are comments on game rules. “Encumbrance stinks! Tenser’s floating disk.” “I hate ambushes! Leomund’s tiny hut.” “1e spell memorization, right? Rary’s mnemonic enhancer.” No Fireball, Charm Person or other staple spells here. These spells are all in the category of “liveable without.”
rare versions of every spells
This proposed rare-spell rule is fine as far as it goes, but I think the rare-spell idea could be expanded to the entire spell list.
Way back in 2011, during 4e, I blogged about adding rare 4e spells: that is, improved versions of common spells, like “Flame Jester’s Improved Fireball” which does extra damage something. I still think this is a good idea. I’d love to see a book full of these: say 3 variations of each common spell, each with a different proper name attached. These would be slightly more powerful than the basic version of the spell, but cast with the same spell slot. Because they’d only be obtainable as treasure (or via costly research), they need not be precisely balanced against other spells: they could be treated in power more like magic items.
Non-wizard classes could get in on the spell-collecting fun. Sure, clerics already have access to the entire Book of Common Prayers represented by the PHB cleric spell list, but there are rare versions of clerical spells only learnable from ancient prayer books, pilgrimages, and the like.
As an example, here are three rare versions of Fireball:
Flame Jester’s Explosive Fireball: For each 6 rolled on the fireball’s damage dice, the radius of the spell increases by 5 feet. This radius increase is not optional. Flame Jester was undoubtedly very amused to be killed by this spell.
Nanda’s Fireball of Death: At casting time, the caster may choose to inflict either fire or necrotic damage. The necrotic version of the spell is a silent black ball of flame that does no damage to objects.
St. Cuthbert’s Holy Fireball: This spell does no damage to anyone wearing a holy symbol of St. Cuthbert.