Originally posted by: Wistfulness
It's a long story. 😆
After Vichitravirya attained adulthood post Chitrangada's demise, Bhishma went to a Kashi swayamvar to get three brides for his brother. He challenged and defeated all assembled kings and brought the princesses to Hastinapura in accordance with norms of viryashukla.
Amba, the eldest one, approached Bhishma and revealed to him that she had already chosen Shalva as her husband and requested him to do what he thinks is appropriate. Bhishma then sends her back to Shalva.
The texts don't mention Amba after this until Udyoga Parva i.e. preparation of war.
Post being appointed the commander in chief, Bhishma narrates the remaining story to Duryodhana when the latter questions his refusal to kill Shikhandi.
After Bhishma sends her back, Shalva rejects Amba by saying she now belongs to another man. Amba tells him that Bhishma doesn't desire her and that he did it for the sake of his brother. But, Shalva still rejects her.
Amba leaves the place cursing her father, Shalva, Bhishma, and even herself. She consults sages and they all conclude that among the three men, Bhishma is the greater cause of her sorrow. Then, Parashurama is approached who is requested by Amba to kill Bhishma. Rama summons Bhishma for a fight which results in a stalemate with the former accepting defeat. He suggests Amba to seek Bhishma's protection but the former desires revenge and leaves.
Amba performs austerities to attain another body and succeeds. She sets a pyre and enters it vowing destruction.
Bhishma then tells Duryodhana that Shikhandi was born a female but was raised as a son. The secret of Shikhandi being a female was unknown to all except Drupada. After the former's marriage to a princess, the secret got exposed and the the father of the princess attacked Panchala. Embarrassed, Shikhandi fled and met a Yaksha in the jungle with whom she exchanged her sex.