Blistering
You have blisters over (IE) × ten percent of your body. If you wear armor, you must wear additional padding to protect your skin from it or lose (IE) hit points per day as it snags, so you have disadvantage on Constitution saving throws in extreme heat as outlined in official fifth edition sources. Because of frequent Fissures in your skin, you must succeed on a DC 5 + (IE) Constitution saving throw every day or develop Infection.
Real-world Examples
Autoimmune Blistering Diseases, Genetic Blistering Disease, Grover’s Disease, Erythema Multiforme, Porphyria Cutanea Tarda, Staphylococcal Scalded Skin Syndrome, Impetigo
Assistive Options
A successful DC 8 + (IE) Wisdom (Medicine) check (with advantage if assisted) will give a +2 bonus to the daily Constitution saving throw.
Magical Assistance
Healing magic applied directly to the blisters will eliminate the need for the daily Constitution check but must be applied daily.