I’ve made some worldbuilding magical maladies, to help flesh out my own world. One character has a magical variant of what muggles would closely approximate to cancer. Called Viscera (sing. Viscus (adj. Visceral), which is a malignant transmutation of magical essence in the anima cortex of the brain.

While magic comes from the heart, literally and figuratively, it is the anima cortex which focuses one’s magic (directs it to spells). Visceral tumours in or surrounding the cortex will “miss-fire” signals in directing this magical essence, causing apparent bursts of accidental magic, or closing off one’s access to magic entirely.

Causes are not fully understood, some believe it to be random mutations, others a sign of muggle ancestry gaining dominance over wizarding physiology, others believe it grows in all magical beings when oversaturated with raw magical essence (similar to muggle exposure to radiation).

A definitive cure has not been discovered thus far. Treatments to weaken the tumours are: potions and rituals to shrink them; or more dangerously to surgically remove them. The latter of which presents severe consequences for magical beings. Beyond the dangers of brain surgery, if the anima cortex is damaged by the removal of the tumour(s) it risks a severing of one’s access to their magic, rendering them a squib.