It seems that you are having linear depressed scar like lesion on fore head.
It may be linear localised scleroderma, also known as en coup de sabre.
Thorough history, keen examination and investigatios along with skin biopsy would be helpful to reach to the diagnosis.
For that ,you must consult dermatologist
Immunosuppressive agents, phototherapy
may be helpful.
But , all under guidance of dermatologist.
I hpoe you got my answer.
Dr. Ilyas Patel MD