The seventh child was transferred to Rohini, the second wifé of Vasudeva. Krishna was the eighth child, who was exchanged with the newly bom daughter of Nand and Yashodha, miraculously, on the night of his birth. The seventh child Balarama and the eighth child Krishna both were brought up by Nand. When they escaped the evil designs of Kansa, the tyrant ordered all the young male children of his kingdom to be killed.
He wanted the killing of Krishna by any rneans, therefore he sent several demons to do the job, but none of them could hann the divine child. Htt invited both the brothers Balarama and Krishna te Matbura, where the powerful demon Chandura was engaged for the job. Krishna killed Chandura, the wrestler and afterwards killed Kansa also. See : Das Avatars Krishna
References :
1. Kohli,Surindar Singh ed,Dictionary of Mythological References in Guru Granth Sahib 1993