The
Key to the Focusing Iris drops from Sapphiron in 10-man Naxxramas. Likewise, the
Heroic Key to the Focusing Iris drops from Sapphiron in 25-man. Each key allows you to start the encounter with Malygos in the
Eye of Eternity on the same mode you obtained the key in. You can also use the heroic key to activate 10-man Eye of Eternity. As a bonus, you can get extra loot for killing Malygos once you have started the quest given by the item.
To begin the question, you must travel to the top of the Wyrmrest Tower in the center of Dragonblight and speak with Alexstrasza and Krasus. They will send you to the Eye of Eternity to defeat Malygos. Once you have done so, you will be rewarded with a choice of one of 4 necklaces. To view the necklaces that you can receive, check out the loot for Eye of Eternity on
10-man under Judgment at the Eye of Eternity and on
25-man under Heroic Judgment at the Eye of Eternity.