Thanks for your query and a clear picture.
This is hard and swollen as the skin is least mobility due to an underlying skull bone.
The appearance of the lesion, as seen in the picture, looks to be like dermatitis and may be due to seborrhea.
The best way is to get an opinion of a Dermatologist as the proper diagnosis is possible due to a clear picture. Further treatment will obviously depend upon the proper diagnosis.
Do nothing at home at the moment before you show this disease to the Dermatologist so that the character remains same to get a proper diagnosis.
I hope this answers your query well.
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