I'm currently DMing a 5e campaign and am thinking of adding some home brew content. One of which is to include the use of magical runes, it's just a thought at the moment but I think it could add a lot to the mechanics of the game. So the idea is that spellcasters could cast at least some of their spells as runes (spells put into word form and activated at a later time due to a trigger) which could be used to lay traps, create intricate alarms etc. and could be used to potentially avoid certain limitations on spells such as line of sight etc. Can anyone forsee a problem doing this? Would it potentially be game breaking? And what rules should I enforce surrounding their use?
Glyph of Warding does most of what you want.
It does have some limitations, such as taking an entire hour and significant material resources to cast, and requiring a spell slot each for the Glyph and the stored spell (unless you go with explosive runes). Part of the reason I suspect for Glyph of Warding being as costly as it is comes down to it allowing you to escape most of the normal limits on spellcasting:
- it pre-loads the action needed to cast a spell which can break action economy,
- as you stated it also allows the caster to escape Line of Sight requirements,
- it lets the caster effectively maintain two concentration spells,
- long rests don't reset it the way they do many other things, meaning you can pre-load it more or less infinitely (with enough time and money) leading to hilarity
- and likely more.
There are other spells that you can effectively drop on an area to be activated later, such as Magic Mouth. Both of these spells have a duration of "Until dispelled", and I suspect so will similar spells. The site doesn't let me browse by spell durations, unfortunately, but if you keep an eye out for either that duration or "Permanent", you'll find more of what you're looking for.
The best part is, it almost completely obviates the need for a homebrew system, so you don't really need to worry about balance if you stick to it, and if you need to fudge something you have these as a baseline.
I think it is a rabbit hole that will be more trouble than it's worth. By making spells castable as runes, you rip apart a lot of the balance that was/is put into 5th edition. When trying to add home-brew rules, it is always best to consider the following:
By making this changes, does it bring value to the experience, or is it just because it "sounds cool"
Converting to rune sounds like, "It was cool in Skyrim, so let's do it."
Given that, (And for full disclosure, I do QA for a living so finding issues comes naturally) let's take a simple spell as an example of what would be required to allow runes:
You cast Magic Missile as a rune at the entrance of a room so if an enemy walks through, it will go off.
- How does the rune determine friend versus foe? Or has the caster locked themselves in? The caster is no longer the aiming mechanism.
- Can the rune be triggered with an inanimate object?
- What is the trigger: Touching? Crossing? Proximity?
- Is the trigger instant or is there a build-up?
- Does casting the rune take up a spell slot? Does the rune vanish at the next long rest or stay until triggered? Could this be exploited as infinite spell casting?
- Can it be dispelled by covering up the rune (Think erasing part of a summoning circle)? Can it be Dodged?
- Can the caster dispel it manually? By dispelling it, do they gain the spell slot back (if it used one in the first place)?
- Can it be Perceived? What's the DC?
Not saying it can't be done, but it's not something to introduce lightly.
As already suggested by @sevenbrokenbricks in comments, but now also in answer form:
There is a spell that does exactly what you want: letting you cast a spell into word/rune form, so you can use it as a trap or delayed trigger.
The spell is called Glyph of Warding. It's a 3rd level spell that is available to Bards, Clerics and Wizards. It has a one hour casting time and 200gp component cost, which suggests that balance wise, making runes is very powerful (and it is) but the Glyph does allow you to use any other spell of an equal or lower spellslot when it triggers. Or it just explodes.