Fiend Fever (5e Curse)
From D&D Wiki
A creature under the effect of fiend fever has their type changed to fiend. They also gain a vulnerability to radiant damage. If the curse is not cured within 3 (1d8 - 1) hours, their skin turns grey and their eyes turn yellow. They also get a disadvantage on Charisma (Persuasion) checks but advantage on Charisma (Intimidation) checks. If the curse is not cured within an additional 2 (1d4) days, their hit point maximum is reduced by 7 (2d6), and they gain claws on their hands and feet, causing unarmed damage to deal slashing damage instead. Finally, they grow a long, thin tail, and their alignment changes to neutral evil. A greater restoration spell cures the curse, preventing it from progressing, and removes the alignment change. All other physical changes are permanent, and can only be reversed with a wish spell or similar.