After the first wing was offered for free at expansion's launch, the second wing of Hearthstone's single player campaign Curse of Naxxramas - Plague Quarter - is now available at £4.99 or 700 gold (same price as every wing that you can buy separately).
This new wing offers three bosses to overcome (Noth, Heigan and Loatheb) and four additional cards (Stoneskin Gargoyle, Unstable Ghoul, Sludge Belcher and Loatheb). You can also buy various bundles depending on what wings you already own (£17.49 for all five wings - £13.99 if you only need four).