The pathway resulting in the formation of N-alpha,N-alpha,N-alpha-trimethyl-L-histidine from histidine. Histidine undergoes three methylations by a histidine-alpha-N-methyltransferase (EC:2.1.1.44) to form N-alpha,N-alpha,N-alpha-trimethyl-L-histidine (also known as hercynine or histidine betaine).