Mahabharat

Devavrut

Shantanu saw Ganga devi and fell in love with her.
She put one condition that he shouldn't ask her any questions.
They got married and had 8 kids.
Each time a kid was born Ganga would immerse them in her water.
Shantanu was tied to his promise and didn't ask seven times. But the citizens started talking and he too was sad that seven sons were killed mercilessly.
He asked her before she could kill his eighth son.
Ganga said that he broke his promise, but told that
Shantanu and Ganga had a curse when they were in Swarga and had to come to earth as part of the curse.
She also said that eight Vasus were cursed to take birth on earth when they tried to steal a cow from a sage. They had requested Ganga to reduce their life on earth.
But since Shantanu asked, one of them had to live the curse.
Ganga named him Devavrut, took him to the best sages for his education and returned him to his father when he was ready.
Devavrut was a very capable prince both in royal/law/political matters as well as warfare.
He and his father once went for hunting.
Shantanu saw a fisher woman, fell in love with her, requested her father to marry her.
The fisherman put a condition that if he promised that her children would ascend the thrown then he would marry his daughter to him.
Shantanu couldn't give his capable sons rightful position to someone who wasn't yet born.
He returned sad.
Devavrut saw his fathers state, questioned his fathers charioteer, went to the fisherman, and requested his daughter to be married to his father. He gave up his throne rights for his fathers sake.
But the fisherman raised a question that if his sons would challenge her daughters sons or their sons, then what?
Devavrut made a promise that he would never marry, and asked him to let him take her to his father.
Since he made such a difficult promise, he was named Bhism.

His father was completely confused at what he did, and gave him a boon to die when he wished to.
Bhism made another promise that who ever is own throne, Bhism will respect him like his father
and protect Hastinapur at all costs.

In future, these few promises made him like a slave. He was respected and had an important position, but many things he disliked happened and he couldn't do anything about them.

Shantanu and his new wife Satyavati started a new life but Shantanu was not happy with Bhisms future, and passed away soon after his wife gave birth to two boys.
Theses boys didn't have good health. The second son died before marriage.
Bhism had forcefully brought three daughters of Kasis King for his brother. But he died before having any sons.
The eldest daughter had requested Bhism to let her go as she was in love with a different king, and he let her go.