A chronic scarring folliculitis affecting the nape and occipital scalp.
Acne keloidalis nuchae, more accurately called folliculitis keloidalis, causes firm bumps, pustules and scar-like plaques at the back of the scalp and neck. It is more common in men with tightly curled hair. Despite the name, it is not ordinary acne and the scars are not true keloids. Repeated close shaving, friction, inflammation and ingrown hairs may contribute. Early treatment can reduce progression; advanced disease can leave permanent scarring alopecia.
DermNet folliculitis keloidalis: https://dermnetnz.org/topics/folliculitis-keloidalis