Building/Adopting In-Game Spellcrafting Systems

[DragonBaneDM]

Hello!

Lots of our games probably include a homebrewed spell or two that our PCs crafted and named after themselves, or sometimes we as DMs come up with a new spell that fits an enemy, NPC, or even as a worldbuilding/plot tool. I'm sure that we've all tried our hands at new takes on an old spells/powers that never got "ported over". A lot of the time, we work with the player to figure out what the spell does, how it ties in with their character, slap a spell level on it, name it "Higby's 1-Man Band" and establish/build upon the norm that powerful, exceptional spellcasters make their own spells in our worlds.

But what I'm wondering is, if you were to go out and make an opportunity for your players to engage with forgotten techniques, eldritch machines, or even just an everyday thing like a magical school/library that let them manipulate whatever Weave/planar power explains magic in your world, how would that look? What do the characters risk by doing this? What important NPCs from your world would have used the same system in the past to make their own spells, and what safeguards would you need in place to explain why one archmage isn't currently lording over your setting because they have 9,999 extremely niche custom spells to tie to a Contingency?

My next writing project in my Eberron campaign is for a megadungeon arc called Skyrock. The details of where and why my party will be able to create their own spells isn't important to the goal of this thread, but I'll summarize what I'm trying to do with the overall idea in a bit. If anyone's familiar with the settings of Eberron and Strixhaven and wants to gut check my lore-ideas, here's how I'm blending the two:

I'm working on a megadungeon arc soon for my Eberron game, and it's based around Keith Baker's ideas for how Strixhaven in Eberron would work (https://keith-baker.com/ifaq-strixhaven/). Essentially, this is where dragons first began to teach "lesser races" like giants, orcs, and elves the basics of magics of all power sources (arcane, divine, and primal), and a cadre of powerful dragons each founded a college based on two conflicting concepts of magic, but that's where the similarities end.

While the adventure of Strixhaven is built to be a Magical Academia story with level 1 characters attending their first Rowling-esque magic classes, Skyrock is meant to be a lost vault of magical secrets, monstrosities, and echoes of powerful spellcrafters from across the eons. The school itself was mostly destroyed and cut off from safe travel by the same tragedy that broke Xen'drik, where it was orbiting at the time.

So this isn't magic school, this isn't even magic grad school (which totally crossed my mind at one point). This is a dungeon in the ruins of what used to be a school that, over the course of it's exploration, will give my party the ability to design their own spells, as well as possibly battlemaster techniques, magic items, and more. I want our Phantom Rogue and Swashbattler Master to be able to participate as well, but the spellcrafting feels incredibly important to get right.


TLDR of all of that is that my campaign's process for designing custom spells/powers/items will be completing a megadungeon. My thought was to tie each wing/miniboss of the dungeon to one or two schools of magic, basically "unlocking" those schools of magic for making a new spell.

However, I'm still kinda clueless on what the "Okay now let's make a spell with what you've learned" part will work, and that's truly where I need help.

I'm hoping that other folks have ideas or resources for long-form skill challenges, down-time activities, or even just systems that improve on what the DMG gives us here. I've seen a few videos and systems online and messed around with them before for single spells, but I'd still like to see what other people's' lived experiences are with setting up a spellcrafting system, explaining the rules to your players, and letting them have at it.

I'd love to look at what's worked well for people in the past, take some options to run by my players, and then shamelessly steal it so that I have to do as little work as possible. make sure that we weigh all of the thoughtful options to maximize the amount of fun our group has together on this new adventure!

[Dualight]

The first thing that comes to mind, for me, is to break down preexisting spells into component properties, that the players can then reassemble into new spells, so magic missile breaks apart into force damage, autohit, 1d4 damage/projectile, and 2+ (slot level) projectiles. Or however else you feel makes sense.
As for the gameplay mechanics of crafting spells, maybe abstract it as a downtime mechanic? have the acquisition of the new spells coma at a cost, so they count towards the spells known count for casters with that limit, while preparation casters overwrite a spell on their list with the new one. Maybe with an exemption for (spellcasting ability modifier) custom spells that can be retained without sacrifice? as for that hypothetical ultra-niche contingencies mage, that is addressed by contigency's own restriction, of 5th level and lower only + 10 day duration. probably a good idea to have a mechanic to introduce drawbacks, like some effects demanding a component, or having a minimal spell level.

Honestly, spells have so many moving parts that making a consistent system is unlikely to be worth the time it would take, at least for crafting custom effects. making a system for hybridising preexisting spells(with restriction like needing to already know spells with all of the mechanics you want) should be more doable.

I have been thinking on something like this (spell crafting) myself for quite some time as well, but when it comes to completely new spells, the best way is probably to allow the players to suggest homebrew they found/made that had not been added to the game in session 0, with approval being on a case-by-case basis. This would also reduce the overhead on the DM as you would not need to plan for possibilities until th players suggest them.

For a narrative restriction, just say that there is a limit to how much power you can squeeze out of even a 9th level slot, so there is a cap n how powerful a single spell can be(even wish has horrendous backlash if you try to exceed those limits), and straight-up declare that some modifications are impossible( such as making contingency both longer-lasting and capable of storing higher-level spells, for example)

This got a bit away from me, but I hope that there is something useful in my ramblings

[Amnestic]

If I were to do this I would steal "adapt" (either directly or parts of) the spellcreation system from Tyranny where you mixed Sigils together. Each spell had up to four Sigils - A Core (spell type) and Expression (Spell area) sigil were mandatory, with Accents (+number bonuses mostly) and Expressions (+rider effects) as optional. In order to craft a spell, you needed a sufficient Lore skill for the sigils you were using, which you could substitute in 5e with an arcana roll (or multiple arcana rolls - say 20 total "successes", each check takes an hour of downtime, and there's no penalty for failure other than dragging it out).

https://tyranny.fandom.com/wiki/Spell_creation

You could acquire new sigils as loot/quest rewards, by buying them, or by convincing people who knew/had them to give them to you.

If I were to add it to a game I'd have it be the casting techniques of a dead empire that everyone just doesn't think is important anymore. Sometimes sigilstones might show up as trinkets in 'civilised' locations but for the most part they'd be treated as pretty oddities, and the mages of the modern era would disregard them as obsolete forms of magic - until, perhaps, the PCs started showing up with them doing incredible things, and causing a cultural shift.