Hearthstone’s next expansion is Reign of the Black Empire, a journey back thousands of years into the distant past when the Old Gods were at war with the Titans. GameSpot has an exclusive reveal for a package of Death Knight class cards, including a big legendary that could swing the game in your favor–or even just win it outright. The three cards are blood runes, which emphasizes control and life-leeching effects. The first is Grafted Knight, a 2-blood rune, 2-mana…