Under the alias of Tobi, the masked member of the Akatsuki is actually Uchiha Madara. For more information of Uchiha Madara, read Naruto volume 43 (chapters 395 to 402).
Madara is the one who trained Itachi, and helped him wipe out his clan. He's the founder of the Uchiha clan, and Akatsuki. He is also the most powerful Uchiha known.