I got this feat idea thinking what I would like my Lore Bard to become. I wanted him to play with dangerous lore, one that could get him hurt in exchange from some power. On the other hand, I do not want him to get hurt badly, or to get power higher than any other character in the party. So I got the idea about the feat, but I imagined 3 different versions of it, and I honestly can't figure out which one would be appropriate. Thus, I'll present you all three and humbly ask to tell me which one you think is most balanced against other character options. Number 2 is my favorite, though.
Prerequisite: Ability to cast ritual spells
You found the truth behind spellcasting and magic in this world. Or, at least, you think you did. When you perform rituals, you can access spells most other casters couldn't use that way, but you risk paying for it with your own vitality. If you don't have a spellbook / ritual book yet, you need to obtain one and it works like one granted by Ritual Caster feat, using the spellcasting ability you already use for your rituals (choose one if you have many), with following exceptions:
The above text is meant to be common for all versions. Now for the differences:
You can copy any spell that does not deal damage from the class list that you can cast your rituals from to your rituals book, even if it lacks Ritual tag. If you cast that spell as a ritual, you must make a Charisma saving throw with DC equal to 10 + spell level. If you fail, you take one exhaustion level. You can't attempt to cast a spell like that if taking exhaustion level would kill you.
You can copy any spell that has ritual tag, regardless of the class list that it appears on, and even if it does not appear on any class list. If you cast a spell that is not on the spell list you would otherwise have access to, you must make a Charisma saving throw with DC equal to 10 + spell level. If you fail, you take one exhaustion level. You can't attempt to cast a spell like that if taking exhaustion level would kill you.
This option works like a Warlock's Book of Ancient Secrets, but worse due to the exhaustion risk. This similarity bothers me, but at the same time suggest it might be balanced, maybe underpowered even.
This would be as both above, allowing both non-ritual spells and spells from other classes, but honestly I think it would be rather much.
Charisma save was meant to represent caster mentally opposing the "great force beyond magic" trying to suck his stamina, rather than purely physical endurance. I am open to changes here.
The actual force that answers to the rituals, and if the character even knows what is it, should be up to player and DM to decide, kinda like warlock patrons. It might be a Shadow Weave in Faerun, it might be some horror from beyond, it shouldn't really matter except for roleplaying most of the time.
I believe that comparison with Ritual Caster feat is unjustified as this feat can only be taken by characters that already have Ritual Caster feat, or class feature that make Ritual Casting redundant and unattractive.
So, which version should make it into playtesting, and what things would need to be corrected before it does? My personal preference is 2, so if you think both are viable, please evaluate 2.