Indented vertical line on my forehead, spreading into more lines. Please help! I have an idea but hoping I'm wrong (Photo)

Hi,
This can be a simple wrinkle that gets worse over time. It all depends on your age as well. Does any of your parents carry same line ??
This also can be one of the very rare diseases , but I would rule out common ones first.
Hope this helps 

Spontaneous onset depression on the forehead  could be a sign of rhombergs syndrome .It is basically  soft tissue atrophy around the forehead and one half of the face. If the depression is small and isolated then nothing much to worry.lipofilling or soft tissue filling is all that you'll need .please get yourself evaluated by a plastic surgeon.

There are several possible explanations for this. However, this could be a thickening of the skin that might be start of what we call en coupe de sabre. I would see an experienced dermatologist and have them get a look in person.