I propose implementing a mental health bar. This bar takes damage when you overcast, regens at a steady rate (which can't be sped up like vanilla health can), and it kills you if it hits 0.

A mental health bar would make "spam-overcasting 19-damage spells" unsustainable even if you have a way to cheese your vanilla health, while still allowing you to overcast in moderation.

Absolutely no gameifying mental health in this mod. But this is an issue we need to fix ... we'll worry about it after the merge, I guess.

Understood. In that case, perhaps the bar could represent:

Integrity (Barrier Integrity, Akashic Integrity, Synaptic Integrity, Hexical Integrity...)
Burnout (Akashic Burnout, Synaptic Burnout, Hexical Burnout...)
Hex Stability/Instability
Over-Enlightenment Risk

I think Math raises some good points, but just to put in a dissenting opinion I think with some fixes the current overcasting mechanic is good, flavorful, very different from other magic mods (which give you a resourceless power bar), and also lowers the stakes of hexcasting.

Overcasting as it stands (with reasonable limits and with correct costing, see #77) opens some fun optimization problems for what we might call "blood" mages. It's possible to cast a spell cheap enough that you can then cast a regen that recovers your costs (typically with a regen 1 for a few seconds). This is simply way cooler than relying on passive recharge rates or an arbitrary bar. Doing this correctly means a handful of free magic, some hitstun and weird particles. Doing it wrong means you die. I think that is cool and fun.

And in the grand scheme of things, no spell you can cast within 19.5 cost is actually that amazing or impactful compared to spells beyond there. It will still be preferable to use media in nearly every case, if only because hitstun immobilizes the player.

The problem with casting from HP is that you need to implement all sorts of random workarounds to solve issues like:

1. Invincibility-frames making it possible to Thoth a 19 HP spell with no issues whatsoever.
2. Other mods giving the player a ridiculous number of extra hearts or damage-reduction or insta-healing.
3. Highly abusable vanilla+mod interactions to regain health absurdly quickly. (For example, having a mod apply Regeneration I for 0:05 seconds, over and over again every tick, effectively giving you half a heart every tick.)

Now, obviously these are solvable problems, but solutions wouldn't be perfect since you're effectively trusting other mods to not fundamentally screw with vanilla HP in any meaningful way. That isn't a safe assumption to make, especially in the large modpacks many players will find Hex Casting in. That's why I'm petitioning for a non-HP-based solution here, despite the points KirinDave brings up.

It'd still be possible to have a system where Doing this correctly means a handful of free magic, some hitstun and weird particles. Doing it wrong means you die. without that system having to rely on vanilla HP. All you need to do is have it rely on some other bar or resource separate from vanilla HP.

Math did get the last laugh here given the current Bloodweave stuff giving people hundreds of dust for free.