One night in Karazhan card back

hello, so there's something that I have to ask. so I want that card back called "Heoric one night in Karazan" and it says that you can get it by completing one night in karazan in heroic mode. I don't have anything purchased regarding karazan, I just won very first thing against medivh on heroic mode and didnt recieve any card backs. so what I want to know is what is considered as "one night in karazan"? plz help