The Plague Quarter has been released in the Curse of Naxxramas Expansion.
In the Plague Quarter you will encounter Noth the Plaguebrigner as the first boss which will award two copies of the Stone Gargoyle Card.
The next boss will be Heigan the Unclean which has an eruption as his Hero Power. Upon defeat you will be rewarded two Unstable Ghoul cards.
Upon beating the previous two bosses you will meet the final boss, The Master of Miasma, Loatheb. Upon his defeat will award you the Legendary Loatheb card.
Just like the previous Quarter, there will also be two class challenges after you have beaten Loatheb on normal that will reward new class specified cards upon defeat.
The next quarter will be the Military Quarter which for me implies skeleton soldiers.