Rune Caster is extremely broken.
Here is why:
Action economy.
By far the biggest problem is that Rune Caster completely disregards the action economy. As written there is no limit to the number of runes I can activate per turn, nor do the runes require any kind of Action to trigger.
For example I could have 10 runes that just require the same trigger word. When I speak that word (no action required) I can unleash an entire combat's worth of spells.
Consider similar options:
- scrolls require an action to use.
- the Ring of Spell Storing requires an action to use.
- the glyph of warding has the action economy problem, but the glyph is stationary so the abuse is very situational.
- contingency would have the action economy problem, if it were not limited to one contingent spell.
Accumulation of consumables.
The second biggest problem is that Rune Caster makes accumulation of runes far too cheap and far too fast and places no upper limit to the number of runes.
5th Edition assumes that the vast majority of a character's powerful abilities are tied to short or long rests, which is not the case for your runes. If you haphazardly sidestep this assumption the game will just fall apart.
Consider similar options:
- scrolls have prohibitive costs both in terms of time and wealth needed to scribe, so unless the DM intentionally gives out scrolls, the party won't have many.
- the Ring of Spell Storing can only store up to 5 levels worth of spells at a time.
- the glyph of warding has accumulation problems, but once again the abuse is very situational because the glyph is stationary.
- contingency is limited to one contingent spell of 5th level or lower, which must be recast every week.
Lack of restrictions
Rune Caster imposes almost no restrictions on the runes, both in terms of who can use the runes and in terms of how the rune's spell targeting works. This means that the runes can basically be used like any other spell. This is not as big of a problem as the ones mentioned above, but it compounds with them.
One way the lack of restrictions compounds with the action economy problem, is that I can share the runes with my party so that we can all break the action economy together. Yay, teamwork.
One way the lack of restrictions compounds with the accumulation of consumables problem, is that the high-level runes are many times cheaper to make than their scroll equivalents (despite being significantly better), so I can make tons of profits by selling runes.
Consider similar options:
- scrolls can only be used by characters that have the scribed spell in their spell list.
- the Ring of Spell Storing lacks restrictions of this kind.
- the glyph of warding restricts the target of the glyph's spell to be whatever triggered the glyph.
- contingency restricts the target of the contingent spell to only the caster.
Conclusion
A pattern emerges when observing those similar options. Scrolls, the Ring of Spell Storing, and contingency each strongly address at least two of those 3 issues in some way. Whereas glyph of warding gets by because its stationary nature makes it really difficult to abuse. When someone does find a way to abuse it, it's not nice.
Gaining Rune Caster is like learning a version of glyph of warding which:
- you can cast at will
- consumes less material components
- has no restrictions on spell level
- has (almost) no targeting restrictions
- is extremely abusable because it is not stationary
How to make a balanced Rune Caster feat
If you want a Rune Caster to be about as good as a feat, you will have to make some serious compromises. When homebrewing, I recommend starting from an equivalent thing that is already balanced.
For example, consider Drow High Magic. This feat lets you cast 3rd level spell once per day among other things. I will caution you that this feat is quite extreme, but we can use it as an upper bound reference for a feat that revolves around glyph of warding, which is also a third level spell. While we're at it, we can borrow some wording from Magic Initiate to keep everything smooth.
Rune Caster
Choose a class: bard, cleric, or wizard. You learn one 1st-level spell and one 2nd-level spell from that class's spell list.
In addition, you learn glyph of warding and can cast it at its lowest level using this feat. When you do so, the only spells you can store the are the 1st-level and 2nd-level spells you learned with this feat, but you do not need to expend a spell slot to do so. Once you cast it, you must finish a long rest before you can cast it again using this feat.
Your spellcasting ability for those spells depend on the class you chose: Charisma for bard; Wisdom for cleric; or Intelligence for wizard.