The biggest, nastiest possible invading forces are those with roots in creation and destruction of life are Old and/or Forgotten Gods. These big, nasty beasties rear their ugly heads (and tentacles and many-fanged maws) in all manner of entertainment media, and they are generally not showing up to make life better.
Old and Forgotten Gods aren’t always quite the same (ignoring the ever-present tentacles in most cases), but they all seem to share the same capacity for inducing madness and serious unrest wherever their presence is felt. Whether it’s their mere existence or a gradual, quiet whispering of dark horrors, any who are unlucky enough to be sensitive to these beings’ existence seem to end up going completely bonkers. In some cases, this could be a form of mind control. In others, it’s simply the effect of gazing into the unknown and realizing there’s some pretty horrifying shit out there.
The key difference seems to be the motivations of these creatures and how they go about handling said plots and plans. The Old Gods lurking about in World of Warcraft delight in bending the denizens of Azeroth, and beyond, to their will, and the chaos and destruction caused in their names is truly horrific stuff. The Titan Sargeras, once a champion of good, and Neltharion, the Dragon Aspect, fell prey to the Old Gods’ terrible designs and became agents of their willful destruction of all life. However, they act as the Old Gods will them to and don’t simply degenerate into forces of wanton destruction, which indicates there could very well be a long plan in play that’s only known to these shadow beings. Also worth noting is their incredible power, as it took multiple titans to defeat even one of the Old Gods, and at great cost. It is suggested that the Old Gods had integrated themselves so thoroughly into Azeroth that their destruction would also mean the destruction of the world. Keep in mind, of course, that my grasp of the lore isn’t exactly perfect here, folks.
Lovecraft’s Old/Elder Gods, such as Cthulu, Nyarlathotep, and Dagon, seem to be agents of the unknown meant to invoke dread and madness in all who are exposed to their influence. Their ancient, eternal status as forces of the universe’s darker side make them terrifying in their absence and presence. I admittedly know little more than the nature of the madness these Gods bring with them, and so I think examples are in order.
Yogg-Saron, N’Zoth, Cthun, Y’shaarj, and others (World of Warcraft) –
So far, only four of the fabled and feared Old Gods in World of Warcraft have been encountered, and each time their influence was felt across both time and space. C’thun and Yogg-Saron’s influence affected denizens of entire continents, inflicting madness in all who heard their whispers. N’Zoth, though unseen, was responsible for the fall of Neltharion (who then became Deathwing) and the events leading up through the Dragon Soul raid–events which threatened the very existence of Azeroth with yet another Cataclysm. Y’shaarj is the interesting one, however, as it shows what happens when these Old Gods fall. Upon its defeat, Y’shaarj splintered into the seven Sha–the manifestations of negative emotions–who wrought havoc over Pandaria while the Old God’s heart remained in a semi-dormant state buried deep in the earth. When Garrosh Hellscream, power-hungry and generally awful, uncovered this heart he actually used the corruption of the Old Gods to further his own strength. It’s an interesting change in the dynamic of how these malevolent beings operate, as typically the Old Gods were pulling the strings and not allowing their powers to be used to further a specific cause.
Tomorrow, folks, is the day I’ve been looking forward to the most, however. Tomorrow marks the rise of the undead, with particular focus on Liches.