Ambaa is a Mahaabhaarat character. Though Ambaa name is very unfamiliar to readers of even Mahaabhaarat, still she is a very important character in Mahaabhaarat, as she was the cause of winning Mahaabhaarat war. If she had not been there, only Bhagvaan knows, how Bheeshm could have been fallen down.

Ambaa was the daughter of Kaashee Naresh. She had two sisters also - Ambikaa and Ambaalikaa. They were married to Vichitraveerya of Hastinaapur. Their father was angry with Shantanu, Hastinaapur king, because he ignored the offer of his sister's marriage to Bheeshm, that is why when he arranged the Swayamvar of his three daughters, he did not invite Hastinaapur king Vichitraveerya. 

His Kul Guru tried to explain him several times that  he should not ignore them but he didn't listen to him and organized  the Swayamvar without inviting them. Bheeshm wanted to marry all three girls to his brother Vichitraveerya. When he did not get invitation from Kaashee,bb he himself went to Kaashee and kidnapped them declaring that he was taking them for his half brother. If anybody wanted to take any one of them, should be ready to fight with him. Now who had the courage to fight with him so everybody sat silent. Even the girls' father could not do anything.

But Ambaa loved Shaalv Kumaar, so when Bheeshm was taking them in his chariot, Shaalv Kumaar followed him and fought with him, but it was his childishness. So he got defeated and came back.

As they entered the Hastinaapur palace, Satyavatee came to welcome them. Ambaa requested Satyavatee, the Raaj Maataa (king's mother), that she loved Shaalv Kumaar and thus couldn't marry her son. Both Satyavatee and Bheeshm readily agreed to send her back to Shaalv, and so they did. They sent her with all respect. But Shaalv did not accept her, so she came back to the king of Hastinaapur and asked for justice that since she was kidnapped at the time when she had Var Maalaa in her hands and she had the right to choose her husband, she should be given the opportunity to do so again. Bheeshm said in the court that "she did not tell us this in her Swayamvar. But when she told us that she loved Shaalv we sent her back to Shaalv, now what?" Ambaa told that Shaalv did not accept her because he felt insulted and now she wanted to marry Bheeshm. 

Bheeshm Jee told that first thing: he did not ask her for himself, but for his brother, so there was no case to marry her. Second thing: otherwise also he could not marry her because he had already vowed to become lifelong Brahmchaaree. At this Ambaa got severely angry and vowed that whether she had to take several births, but she would take her revenge from Bheeshm. She would be the cause of his death. [This was the foundation of Bheeshm's death.] Since Bheeshm did not do anything wrong, he got silent on this.