What can be done for this vertical dent in my forehead, it can only be seen in certain lights. (Photo)

Botox treats lines or wrinkles of facial expression in the skin by relaxing the muscles that cause them.  What you have described is a dent or contour depression of your central forehead. Botox would not be an effective treatment for this condition.  Vertical depressions in the forehead may be caused by the shape of the frontal bone, trauma, low body fat, or much rarer conditions such as linear scleroderma. Sometimes pressure from a large central vein in the forehead can result in a loss of fat  over time due to coughing, laughter, sneezing or exercise.  In pale skin individuals such as yourself,  you can often see blue-tinged veins under the skin. The dark color may lend the appearance of a dent or depression in certain lighting. If you do have a contour depression it can be treated with a hyaluronic acid filler or fat injection. You should consult with an experienced physician injector, familiar with the vascular anatomy in this region, as injections in this area carry a higher risk of serious complication. My best.

you have a condition a form of localized morphea En Coup de sabre. it is a rather a clinical diagnosis and in most cases u do not need a skin biopsy unless there is evidence of other areas being involved. I have treated this condition in the past and another version of the same condition but with rather aggressive and wide spread involvement. u can see some of those pics on my website of Perry Romberg Syndrome. it can be improved upon significantly. 

Hello and thank you for your question.  The
best advice you can receive is from an in-person consultation.  Based on
your photographs, you may benefit from botox.  This decision should
be based on a detailed discussion with equal input from both you
and your surgeon.   I recommend
that you seek consultation with a qualified board-certified plastic surgeon who
can evaluate you in person.
Best wishes and good luck.
Richard G. Reish, M.D.  FACS
Harvard-trained plastic surgeon