In reference to the March 20, 2012, Banned and Restricted Announcement.
Innistrad Block Constructed
Block formats have a significantly smaller card pool than Standard. They tend to be about half as large. As a result, it is easier for a strong deck or strategy to go unanswered, as there are fewer potentially competitive strategies.
The DCI examined the current block format and found that creature-token based strategies were dominating and largely competing with other creature-token based strategies. In particular, Intangible Virtue and Lingering Souls were very prevalent. There were some variations to the base white-black deck. One could add green mana, which adds Gavony Township, gives Ray of Revelation flashback, and allows the option of Garruk Relentless. One could add red mana, which adds Hellrider and Devil's Play. However, this isn't the diverse format most people enjoy.
In the interest of making Innistrad Block Constructed a more diverse and healthy format, the DCI is banning Intangible Virtue and Lingering Souls.