hello friend...you are suffering from periorbital melanosis
..it is multifactorial and the exact cause still remains a mystery..However on avoidance of stress,a good sleep of about 6-8 hours at night and taking plenty of fluids will provide some relief..it can be treated by topical vitamin k and retinoid
preparations used as a combination therapy..you may need laser treatment
if it is resistant to topical treatment alone..kindly meet a good dermatologist
for the treatment..wishing you good health.