The Plot Thickens
Below is the entry from the DMG
Consider using the Sanity score if your campaign resolves around entities of an entirely alien and unspeakable nature, such as Great Cthulhu, whose powers and minions can shatter a character’s mind.
A character with high Sanity is level-headed even in the face of insane circumstances, while a character with low Sanity is unsteady, breaking easily when confronted by eldritch horrors that are beyond normal reason.
Sanity Checks. You might ask characters to make a Sanity check in place of an Intelligence check to recall lore about the alien creatures of madness featured in your campaign, to decipher the writings of raving lunatics, or to learn spells from tomes of forbidden lore. You might also call for a Sanity check when a character tries one of the following activities:
- Deciphering a piece of text written in a language so alien that it threatens to break a character’s mind
- Overcoming the lingering effects of madness
- Comprehending a piece of alien magic foreign to all normal understanding of magic
Sanity Saving Throws. You might call for a Sanity saving throw when a character runs the risk of succumbing to madness, such as in the following situations:
- Seeing a creature from the Far Realm or other alien realms for the first time
- Making direct contact with a mind of an alien creature
- Being subjected to spells that affect mental stability such as the insanity option of the_symbol_ spell
- Passing through a demiplane built on alien physics
- Resisting an effect conferred by an attack or spell that deal psychic damage
A failed Sanity save might result in short-term, long-term, or indefinite madness, as described in chapter 8, “Running the Game.” Any time a character suffers from long-term or indefinite madness, the character’s Sanity is reduced by 1. A greater restoration spell can restore Sanity lost in this way, and a character can increase his or her Sanity through level advancement.
My take on Sanity is that it is based on aberration creature types, as well as some of the monstrosity types with bizarre anatomy, demons, devils, dragons, a kraken, the Tarrasque, that some of thing. I also figured it would be tied to/important for elves and their tenuous grip on “reality”.
As greater restoration recovers all Sanity loss, perhaps lesser restoration has a chance to restore some, maybe 1d4-1, or 1d5-2, I’ll need to check the spell level difference and material costs. I was also thinking that maybe the greater restoration is part of the elven Focus ritual, and that casting either version of the restoration spell could have side effects if not part of the Focus ritual for elves.