Curse of Gandhari
When the war of Kurukshetra ended ,there was heavy destruction everywhere. The whole clan of Kurus was destroyed except the Pandavas and few other warriors . When Gandhari came to know that all of the Kauravas died in the war, she considered Krishna to be the cause of all the deaths and destruction. She blamed Krishna that he killed her sons with cheating and deception.
Under the influence of her anger Gandhari cursed Krishna that his entire Yaduvansh would be destroyed in the same way with tricks, none of the remains would be left . Neither Krishna nor Balrama would be able to save their clan from destruction .
Disapperance of Balram
As a result of this curse , the Yaduvanshi people were influenced by wrong activities like cheating , drinking, prostitution ,gambling. As a result there started to be internal fights among the people of Yadav clan, which slowly converted into large ones and became the cause for the destruction of entire clan . When Balram realised all this he understood that the end was near and he had completed his work of this avatar. So he left his body and converted into his original form as Shesanaga.(Balrama was the avatar of Sheshanaga in the Dwapara Yuga just like Lakshamana in Treta Yuga ).
According to Vishnu Purana:
The disappearance of Balrama from earth was witnessed by Krishna and his charioteer .While they were seeing, a thousand faces serpent came from him and had reached Rasathala, as welcomed by ocean God.
Disapperance of Krishna
After the departure of Balrama ,Krishna urged to leave the earth as he had completed his work as the avatar of Vishnu . Once when he was sitting under a tree ,a hunter saw the feet of Lord with Lotus Sign below and mistook him to be an deer. This hunter was none other than monkey king Bali reborn as the hunter whom Lord Rama had killed from behind the bushes. The hunter shot an arrow which hit the Lord in the feet. Realising his mistake the hunter came to Lord Krishna and asked for his forgiveness. The Lord said that it was destined to be this way. Thus, Lord Krishna left the earthly abode marking an end to Dwapara Yuga.
This event is known as Krishna’s Niryana. There is a temple in the place of Shree Krishna Niryanam in Prabhasa teerth in Gujarat and nearby there is a place called Balramji ki Gufa (the cave of Balram). here we can find the mark of Aadisesha depicted on the cave wall from where he said to have departed. This place is adjacent to the place of Sri Krishna’s Niryana. There is a place, Gopi Thalav, near Bet Dwaraka, Gujarat. All Gopikas had fallen into this lake, after Sri Krishna Niryanam.