The Focusing Iris
Keys to the Focusing Iris are held by the most ancient members of the blue dragonflight. At minimal power, they can be used it to open the iris a fraction, drawing Malygos's ire.
Thrall: Twilight of the Aspects
Malygos had used his own blood to activate and control this orb, which had lain dormant for millennia. With the open Focusing Iris, Malygos had been able to direct (and create) powerful surge needles, using them to pull arcane magic from Azeroth's ley lines and channeling that magic into the Nexus. The slain Arygos' blood, he being a son and direct blood relative of Malygos, was used by Aedelas Blackmoore to use a surge needle (with energies flowing from the Nexus) to animate Chromatus.
It was previously mentioned that once harnessed, the energies are focused through the Nexus' ascending rings and blasted into the Twisting Nether. In Thrall: Twilight of the Aspects, this function of the Nexus is not mentioned. It is implied that energies are stored there.
The Demon Soul
This section concerns content exclusive to Cataclysm.
As part of the Dragon Soul raid introduced in Patch 4.3, adventurers have to get the Focusing Iris so the Dragon Aspects can refocus their power into the Dragon Soul to use the Demon Soul artifact against Deathwing.
Jaina Proudmoore: Tides of War
Reflecting on its misuse in the past and its potential for future misuse, Kalecgos ordered the Focusing Iris moved to a more secure location - encased in enchanted ice, magically warded, and dumped at the bottom of the Frozen Sea. However, the dragons sent to escort it were ambushed and killed by members of the Horde, who stole the Focusing Iris and used it to construct an enormous mana bomb to obliterate Theramore Isle.
While walking through the ruins of her city, Jaina Proudmoore discovered the Focusing Iris at the bottom of the blast crater, and decided to use it to exact vengeance on the Horde. Studying a book of powerful magic originally discovered by Antonidas, she went to Fray Island and used the Focusing Iris first to summon thousands of water elementals, and then to bind them together into a massive tidal wave with the intention of destroying Orgrimmar. After being talked down by Thrall and Kalecgos, she returned the Focusing Iris to the latter, who decided that the safest place for it would be in the Kirin Tor vaults in Dalaran.