Disease: Arcane Fever

Arcane fever is a unique type of affliction that affects powerful and potent casters, as well as those who may be subjected to such power.

Often called "Brain Rot" by many spellcasters, or "Mad Mage Syndrome" by many commoners who have either seen it action or know of someone who suffered from it.

There are many things that can cause a creature to become afflicted with Arcane Fever, though it is typically through contact of Mystic Arcana.

There is no known cure for Arcane Fever, only suppressants, that control it. This disease typically manifests itself in casters at later stages in the caster's life, due to their constant contact with the arcane, and the eldritch.

Contracting Arcane Fever. As mentioned, Arcane Fever manifests itself much later in a caster's life, or if the caster comes in contact with mystic Arcana, or powerful Arcana. Arcane Fever can also affect those who may not even be magically adept, but if they are subjected to, or witness very powerful magic.

As it stands, there are four common ways to contract Arcane Fever: Casting high level magic, being subjected to (or witnessing) high level magic, drinking potions, and witnessing or being near Mystic Arcana.

Casting High Level Magic. There is a reason not too many archmages, and liches exist. Casting powerful magic has it's consequences. Whenever a creature casts a spell of 5th level or higher, the creature makes a constitution saving throw, DC 10+ spell's level. On a failure, the creature contracts a mild case of Arcane Fever. On a success the creature resists the disease.

Subjection To High Level Magic. When a creature is hit with high level magic (and survives), or perhaps witnesses a creature casting high level magic, The Creature must make an intelligence saving throw DC 10+ spell's level. on a failed save, the creature contracts a mild case of Arcane Fever. On a success the creature is unaffected.

Potions. Drinking potions also have a chance of a creature contracting Arcane Fever, even a simple healing potion.When a creature drinks a potion, after the effects end, the creature makes a constitution saving throw, DC 10+ potion's rarity (Increase the DC by 1 for every step above common). On a failed save, the creature contracts a mild case of Arcane Fever. On a success the creature is unaffected.

Mystic Arcana. Perhaps the most common spread of Arcane Fever for the curious mage wandering a ruin and stubling upon an artifact they do not understand, or perhaps just seeing something they do not fully understand.

Mystic Arcana permiates itself across the planes, and multiverse, from eldritch artifacts lying deep within a sealed dungeon from eons passed, to perhaps ancitent runes written on the walls, or equations that one does not fully understand.

Many of these things have caused even the most genius mages to go mad, trying to decode such incomprehencible Arcana.

When a creature is subjected to Mystic Arcana, they must make a DC 20 Intelligence saving throw, on a failed save the creature contracts a moderate case of Arcane Fever. on a success the creature is unaffected.

Symptoms. Arcane Fever can be broken down into three cases; mild, moderate, and severe. Symptoms vary or worsen depending on severity. If a creatures that is already afflicted by this disease is required to make a saving throw against Arcane Fever again, on a failed save, it's condition worsens.

Mild. Creatures with a mild case of Arcane Fever often have a desire to want to try newer and more powerful spells, or have spontaneous bursts of energy. Sometimes manifesting themselves in small arcane effects.

Moderate. Creatures with a moderate case of Arcane Fever have a strong urdge to want to use powerful magic, frequent bursts of energy, usually arcane. A mild Fever sets in by this point, and the creature begins to show a state of heightened Intelligence.

Severe. Creatures have an uncontroablle urdge to cast spells, and almost always seem to have constant bursts of energy. The fever worsens, causing the creature to perpetually sweat. The Creature's already heightened Intelligence swells to incomprehenciable levels and has a strong urdge to study Mystic Arcana. If the creature is ever subjected to another saving throw against Arcane Fever and fails, the creature dies.

Cure. As mentioned, there is no known cure for Arcane Fever. There is however, suppressants for Arcane Fever. These details are intentionally left obscure for you as the DM to create your own solutions to developing a cure or suppressant for Arcane Fever.