The first ever curse that Bhishma was given was actually in his previous birth as a Vasu known as Dyu (or Prabhasa), due to which he had to take birth in the mortal world in the first place!
In this post, we will find out what exactly had transpired...
Once there was learned sage named Vashishtha who owned a cow Nandini, daughter of the wish fulfilling cow Kamadhenu. The cow was stolen by the Ashta Vasus, who are the eight elemental god's representing the eight aspects of nature. It so happened that the wife of Dyu (or Prabhasa), one of the Vasus was attracted to the cow and wanted it for herself. Blinded by the love for his wife, Dyu planned the abduction with support from the seven Vasus. However, Vashishtha found out who the culprits were, through his divine powers. He cursed the Vasus to lose their divinity and be born as mortals on earth. The Vasus pleaded with him to undo the curse. Vashishtha finally relented and modified the curse. The seven Vasus who helped in the stealing would be free from earthly life very quickly, without having to spend much time on earth. However, Dyu, being the main culprit would have to pay the full price and live a long mortal life, to atone for his mistake. However, his life would be an illustrious one and he would be known as a great man. Saying this, the sage walked away.
The Vasus then approached the river goddess Ganga with their plight and requested her to become their mother on earth. After a while, Shantanu, the Kuru king of Hastinapur fell in love with Ganga and asked her for marriage. Ganga agreed on one condition that he would never question her actions. It was a happy life for the two until Ganga gave birth to their first child, whom she promptly drowned in her waters. And this continued for the next six children, thereby liberating the seven Vasus from their curse. At the birth of their eighth child, the king could contain himself any longer and questioned her actions, breaking his promise. Ganga told Shantanu about the tale of the Vasus. She then spared the eighth child and left Shantanu behind. She called this child Devavrata, who grew up to become the great Bhishma!