If using tretinoin 0.05% has caused your under-eye area to darken, it could be due to irritation, as the skin in that area is very delicate and sensitive. It's best to discontinue use of the product and allow the skin to heal. Since under-eye pigmentation can have various causes and requires a gentle, targeted approach, it's essential to consult a dermatologist. They can recommend a safe and effective treatment tailored to your skin type. Avoid using strong creams near the eyes without consulting a doctor, as they may cause more harm than good.