Wrath of the Lich King has heavily hinted at some sort of connection between Yogg-Saron and the Scourge, though we never quite find out what it is. The Scourge make heavy use of Saronite, an ore with Yogg-Saron himself as its origin. When the Alliance investigates this ore in the Dragonblight, they discover the Scourge says Yogg-Saron's name with some degree of hate and contempt.
A lot of people fear there's a "puppet of the master" thing surrounding the Lich King, that he's not actually his own power, he belongs to Yogg-Saron. I find this unlikely for one really big reason: The Scourge hates Yogg-Saron, and everybody else that we've run into that have fallen under Yogg-Saron's will either love him utterly or serve him faithfully. If you go to the Saronite Mines in Icecrown and do the quest Slaves to Saronite, you see just how faithful some of his servants become once they've been controlled. If Yogg-Saron were controlling the Lich King, we'd see a very different Scourge.
This doesn't mean that the Lich King doesn't get any power from Yogg-Saron, however. It's possible the Old God is having some of his power siphoned off by the Scourge. From a story standpoint (and ignoring game mechanics), if a Mage were to find and utilize Azeroth's ley lines, all of that extra, loose magic flowing by could give that Mage's spells a whole crapload of extra kick. The Lich King's throne is more or less directly above the Old God of Death and his domain. We don't know if that was intended by the Legion or purely coincidental, but it leaves a lot of potential for the Lich King to have more power over the Dead than he normally should.
In Ahn'kahet we see the Scourge and Yogg-Saron's servants at odds yet again. The Scourge hasn't fully taken the city, they're only seen in the upper levels of the Old Kingdom. Yogg-Saron and his servants (the Faceless and the Twilight's Hammer) hold the lower levels of the Old Kingdom. When us adventurers break through the Scourge line from behind, then go down into the depths and meet Yogg-Saron's Herald Volazj, the guy says some very interesting things to us. Volazj has quotes such as:
- "They do not die; they do not live. They are outside the cycle."
- "They who dine on lost souls know only hunger."
- "Gaze into the void. It is the perpetuity in which they dwell."
- "Where one falls, many shall take its place."
They've broken the cycle. Their existence is perpetual; endless. It's not particularly satisfying, either. The books in the Death Knight starting zone tend to describe a Scourge lifestyle as somewhat utopian. At least, that's the ideal that Kel'thuzad and the Thuzadin have pushed as their doctrine. It's a life without riches, without racial boundaries, but how many Undead have we met that are happy? They all have needs, they all have hungers that they can't satisfy. Herald Volazj's words describe the Scourge pretty well.
I don't think we're seeing a 'puppet of the master' situation here at all with Yogg-Saron and the Lich King. The two are at odds with one another, the narrative has made that pretty clear. If there's a relation, I suspect it's Yogg-Saron that serves the Lich King, though not fully, directly, willingly or happily.
Patch 3.1 brings us Ulduar, dual specs, significant changes to all the classes, and more! We've got you covered from top to bottom with our Guide to Patch 3.1.