Would allowing wizards to use custom spells break the game?

Question

I'm really fond of the notion of ancient wizards locked up in their towers and conducting innovative research into the arcane arts. Given how vast and ancient the Forgotten Realms are, I get the feeling that a lot of such investigations must have been conducted. However, even though the list of existing spells might seem quite massive from our point of view, I get the impression that you could fit everything there is to know about them in a book of two - and yet wizards have vast libraries and life-long careers. Additionally, in cases when spells at a similar level of difficulty can be directly compared (i.e. damage dealing spells at the same level), their efficiency is suspiciously similar.

An explanation that I came up with for this state of affairs is that there are a lot more known spells in the world, and the ones we see come from a carefully selected list containing only the most efficient uses of magic. After all, if your life and death depend on casting a spell, you're normally not going to settle for second-best. If you really want to, you can carefully go through all the books in the library in search of whatever spell strikes your fancy, but you should be aware that it's almost certainly going to be inferior to the popular ones.

For instance, at the time of writing, DnDBeyond lists spells named Wall of X, where X is any of: Sand, Water, Fire, Force, Light, Stone, Ice. But surely there is also Wall of Water hidden somewhere, except it's as hard to cast as Wall of Ice and ostensibly has less uses. There is also Wall of Lightning, except it's as difficult as Wall of Fire and deals half as much damage. If you look really hard, there's even Wall of Pebbles, which is the worst of both worlds between Wall of Sand and Wall of Stone. Probably there is a thick tome entitled "On Walls" that most libraries have a copy of, with optional bookmarks for the wall-related spells that actually have practical applications.

For the sake of concreteness, let's restrict attention to damage dealing spells. I would like to add the following principle:

For each spell that deals damage there exists an equivalent spell that is identical in all aspects except it deals damage of any other chosen type and deals 25% less damage (e.g. an Iceball that's exactly like Fireball except it deals 6d6 damage)

This related question mentions a similar feature called Spell Secrets introduced in an Unearthed Arcana Wizard subclass called Lore Master, which allows for a very similar effect, except without any reduction in efficiency.

I don't expect that this would be frequently used. The main applications I see are:

• creation of flavourful characters that lean heavily into a specific damage type;
• creation of optimised characters that rely on specific damage types;
• overcoming resistances or exploiting vulnerabilities of enemies;
• making the magical world feel a little more full and fleshed out (subjective).

Is this in some way unbalanced and/or exploitable? Each individual new spell added this way is, almost by definition, underpowered. However, I'm concerned if this change could be bigger than the sum of it's parts. For instance, the ability to use Wish to copy spells becomes somewhat more versatile (although Wish is already powerful enough that probably this is not a big difference). Is there anything else that might lead to problems?

Answer 1

This would significantly alter strategies for fighting certain opponents.

Demons are immune or resistant to cold, fire, lightning, and poison. Devils are immune or resistant to cold, fire, poison, and occasionally acid. Only one creature in the monster manual resists force damage. (See: What are the most and least-resisted damage types?)

We should expect a wizard who frequently battles fiends to learn force-damage versions of all their attack spells. Actually most wizards would probably learn a few such spells.

You've asked whether it would be "broken", and maybe it wouldn't be. Maybe the wizard forced to fight demons is in a really bad spot, and giving that wizard access to forceball spells is just restoring balance. On the other hand, maybe those demons are balanced with lower hit points under the assumption that they'll be harder to hurt.

But I think our definition of "unbalanced" involves whether everyone would want to do a certain thing. And I think probably a lot of wizards would.

Answer 2

You basically just reinvented the wheel, there are vastly more spells than players have access to and more than are provided in official material. look at the actual description for spells in the DMG.

Uncounted thousands of Spells have been created over the course of the multiverse’s History, and many of them are long forgotten. Some might yet lie recorded in crumbling spellbooks hidden in ancient ruins or trapped in the minds of dead gods. Or they might someday be reinvented by a character who has amassed enough power and Wisdom to do so.

This is also part of the justification for why NPC's can use magic the players cannot.

The ones players are given are the ones commonly known, or easy to discover. For balance reasons you don't want to let players create better spells but creating spells of equal value is fine. Quite commonly my players learn NPC spells that are powerful but too difficult for a lone spellcaster to cast, they either take too long or require special materials or extra personnel. I also encourage players to create spells , this involves some back and forth to maintain balance and usually requires some investment of time and resources to research and test the spell. A custom spell can even be a great reward for a spellcaster.

My players saved the assistant of a very powerful but dead spellcaster who was frozen solid, It was very hard but they came up with some creative uses for spells. As a reward the apprentice taught them two custom spells related to his masters elemental cold research.

I often give NPC wizards/druids spells the players do not know about that take too long to cast to be useful. like a weather control spell that requires a week to cast, or a well water purification spell that has to be cast every day for a year before it kicks in but then can be renewed with only a single casting per year, essentially becoming permanent. This is to me why every small town has a wizard or spellcaster, and why they don't want to leave said town. These are spells that are basically useless to players.