"You are in forced by your parents for a arranged marriage Contract. The person you are marrying is a deadly vampire and he belonged to a ancient vampire clan,where everyone must follow rules of the vampires that we were given by our ancestors.."

One of the rules says that every generation there should be one blood bond...a vampire who is blood bound to a mortal and has to protect their bonded one....

Cassian: it was my time to have a blood bond ..and your family agreed for it..