Yggoa, the Crystal Bark
Yggoa is a psychoactive agent that expands the memory. It is highly addictive. Long-term use leads to the formation of mnemonic cavities that permanently impair a spell-caster's overall ability.
The drug is harvested from a rare crystal tree type that is difficult and expensive to obtain. These plants are for the most part cultivated by powerful foreign cartels. When a tree reaches maturity, its crystalline bark is carefully stripped and ground into the fine, luminescent powder called yggoa. Often the pure powder is "cut" for market with an additive called jinth that increases its addictive qualities.
When a magic-user uses a single dose of yggoa, his memory is enlarged to contain 1d8 more spells of his choosing. Alternatively, treat the roll as exploding. A result of "8" indicates 1d8 more spell slots are realized.
Each dose used in this way carries a 5% cumulative chance of causing a mnemonic cavity to form. A cavity effectively drops the magic-user's spell storage capacity by one experience level. The magic-user then returns to 0% chance of mnemonic cavity formation, but every dose of yggoa thereafter increases the risk to 10%. These can even extend (1d30% chance) into the character's long-term memory, erasing the PC's access to a key bit of knowledge determined (openly or in secret) by the DM.
Extreme yggoa use leads to a total black out of spell slots. In these cases, magic-users become totally reliant on yggoa to memorize spells. Yggoa-addicts often appear gaunt and listless. Their eyes are often rimmed with a green discoloration.
Withdrawal is taxing on the Constitution. Following the grace period of three doses, every new dose of yggoa has a 1-in-4 chance of causing a habit to form. Habits require daily intake. One point of CON is permanently lost each day without a dose of yggoa. Habits may only be cured through (non-lethal) exposure to dragon's fire.