The nights king is the 13th lord commander

[Uber40]

It fits in with the legend that the 13th lord commander was lured by a woman whom he chaced and then in my opinion was tricked and gagged before becoming the nights king. Probably one of the COTF used magic to transform into this woman who stayed with him at the castle for the next 13 years. He spent his time converting new WW by sacrificing as he did in earlier seasons and later went into deep hybernation when he was kicked out of the castle.