Arcane monster spell that reduces your memory of a single spell by a few thousand turns.

The number of turns you can remember a spell is directly dependent on your Intelligence, and possibly Wisdom (at the time of reading the book). The default 20000 turns is a baseline for maybe 12 Int and 12 Wis; more of those means you get more turns when you learn the spell or refresh your memory, and less of those means you get fewer turns.

It’s a problem that utility spells with a high failure rate, such as identify at 95% fail, is a mere inconvenience to players, since they can sit in a closet with a stack of food and wait for their Pw to recharge until they succeed. This is not really a problem with combat spells, since the penalty for failing to cast the spell correctly is a disadvantage in combat, where time and Pw matter. Some possible ways to address it:

• Large penalties for failing to cast a spell, like a 1% memory loss and 50 hunger.
• Failing to cast a spell locks it for a certain period of time. It would be hard to make this not turn into an even slower grind for utility spells, since the lockout period could just be waited out.
• Make spells never actually failable, but Pw cost is increased proportional to the failure rate, specifically: real Pw = base Pw / success rate. Under this formula, a 15 Pw spell at 95% fail becomes a 300 Pw spell at 0% fail. This means the Pw cost should be exposed in the spellcasting menu rather than failure rate, even though failure rate still needs to be calculated. Maybe use different colors in the spellcasting menu to denote spells you can cast now, spells you can cast by waiting to recover more Pw, spells you can’t cast even at your current maximum Pw, and forgotten spells.

When spells are forgotten or retention is damaged when you suffer amnesia, the game prints a message informing you of such, akin to “You forget some of your training” for skills.

Instead of spellbooks having 4 read charges, they store “400% memory” - enough magical power to give a reader full knowledge of the spell 4 times over, which is functionally identical to how many times they can be reread now but more granular. Reading a book to restore memory of a partially forgotten spell only consumes the missing fraction of memory required to bring you back up to 100% retention.

Example: The hero reads a spellbook of knock, bringing themselves to 100% memory and the spellbook down to 300% memory. 10000 turns later, the hero, now with 50% memory, rereads the book. This restores their memory to 100% but the spellbook is only decreased to 250%.

This would allow events that degrade spellbooks (reading it and polymorphing it) to degrade it by a randomized degree instead of a flat, predictable 1/4 of its use every time.

Once you have learned your role’s special spell, you never forget it.

Spectres should have some method of stealing your spell knowledge, then gaining the ability to cast those spells themselves.

Casting spells decreases your spell memory by a small amount but one that would be significant if you cast the spell heavily.

A spellbook’s level determines how long the spell stays remembered after you read it, rather than having every spell be remembered for 20000 turns exactly. Lower level spells are remembered for longer than higher level spells.

Some polyform (perhaps mind flayer) that doesn’t forget spells over time while you remain polymorphed into it.

Amnesia makes you forget spells and lose skill points instead of forgetting discoveries and maps. Normally, spell recall is drained; if a spell is at 0%, it is completely forgotten.

Drinking booze reduces your memory retention of spells.

When you forget a spell, give the player a message “You have forgotten the ‘foo’ spell.” Also give warnings when you reach 10% and 1% memory.

The game should allow you to reread a spellbook and relearn the spell whenever you want. The prompt should be “You know [spell] quite well already. Read the book anyway? (yn)”