Devavratha became Bhishma after he took bhishana pratigya ('terrible oath') — vow of life-long celibacy
https://www.google.com/search?q=Shantanu+Naidu
https://www.google.com/search?q=bhishmaiks
https://www.google.com/search?q=Bhishma+Confession
https://www.google.com/search?q=Bhishma+Draupadi
https://www.google.com/search?q=Pandavas+Krishna+Bhishma
https://www.google.com/search?q=Bhishma+Afflicted
