Devwrath who in the latter stage was known as Bhishma was very talented in all types of warfare. He excelled in all archery, wrestling etc. He learns a very powerful weapon known as Brahmaastram from Sage Parasurama.
Shanthanu who is the father of Devwrath gets married to Satyavathi and has two sons Vichitravirya and Chitangadha.
Devwrath arranges the marriage of these two with Amba, Ambika and Ambalika the daughters of the king of Kasi.
Amba refuses to marry saying she wants to marry the king of Salva. Devwrath lets her go to the king of salva.
He gets Ambika and Ambalika married to Vichitravirya as the other son of Shantanu died at a young age.
Amba returns back to Bhishma saying that the king of Salva refused to marry her as she was taken away by Bhishma and requests him to marry her. Bhishma refuses to marry her saying the that he has taken vow not to marry.
Amba gets angry as she refused by two kings and goes in to penance and appeases Lord Shiva and asks for a boon to kill Bhishma due to whom her life got spoiled.
Shiva says that she cannot kill Bhishma directly as he a boon from his father to die whenever he wishes .But Shiva tells Amba that she can be the cause for Bhishmas death and tells her in the next birth she will be born as a daughter to king Drupada and later turn into a boy with name Shikandi and will cause the death of Bhishmna.
Vichitraviya has two sons Dhrutharashtra and Pandu. Druthrashtras sons were known as Kauravas and the sons of Pandu as Pandavas
Bhishma engaged the best teachers to teach these children the best warfare lessons and other lessons.
Duryodhana the eldest of the Kauravas was always jealous of the Pandavas and was always troubling them as he was a person of evil character. Duryodhana always hatches plots and troubles the Pandavas. In one act he sends them to forest and refuses to give them their kingdom.
This leads to the Kurukshetra war where Bhishma is forced to fight against Pandavas even though he knows that Pandavas are right and are on the side of Dharma. He does so because he takes a vow to protect the Kurudynasty at all costs and in the end he was killed with the help of Shikandi.