"Not pretty" is an understatement.
Toxic epidermal necrolysis (TEN) is the most lethal on a scale of 4 related necrotic skin conditions[1]: TEN, SJS, EMMajor, EMMinor. The medical industry sees all 4 as kind of a sliding scale, similar to how burns are classified as different degrees and can be clinically distinguished but are still burns. They are categorized based on the percentage of your skin that becomes necrotic, and additionally which tissue types are affected.
My understanding is that herpes is one of the most common causes, although maybe that's only true for EMM and not TEN/SJS. For some reason, as we age, sometimes our body changes its reaction to herpes or other infections, becoming hypersensitive, resulting in one of the 4 previously mentioned necrotic skin conditions.
I'm sharing this, because even though my symptoms were literally staring the doctors in the face, it took years before a studious dermatologist figured out I had recurrent EMMajor. If you have "cankers" or "rashes" associated with your herpes outbreaks, ask your doctor about it, even if the rashes aren't painful. Recurrent EMMajor can be life-threatening without treatment[2] and can progress with further attacks.
[1]https://www.ncbi.nlm.nih.gov/pubmed/21873140
[2]https://www.nhs.uk/conditions/erythema-multiforme/ (warning: disturbing pictures)