The Truth Revealed: House Lannister is Behind the Doom of Valyria
Based heavily on this genius reddit post:
https://m.reddit.com/r/asoiaf/comments/2l64xt/spoilers_all_the_doom_of_valyria_explained/
In the world of Game of Thrones, before the unification of the Seven Kingdoms under Aegon the Conqueror, the Ancient Rome of that world, which will henceforth be referred to as Planetos, reigned in the eastern continent of Essos. This was the Valyrian Freehold, based on the ancient Roman Republic.
The Valyrians, their dragons, and their magic brought Essos to heel. (More specifically Western Essos, with the Far East dominated by the China-esque Golden Empire of Yi Ti.) But why did the Valyrians never invade Westeros? Apparently the Valyrian sorcerors had a prophecy that gold from the Westerlands would destroy Valyria. They knew the Casterly and later Lannister families had lots of gold and never moved to contact with them, so greatly was this prophecy respected.
But still, the Lannisters brought the gold to them. Shortly before the Doom the Lannisters commissioned Brightroar, their great Valyrian steel greatsword, and they pay for it entirely in their native gold. It is said multiple times that they overpaid heavily, giving up so much gold for that blade that they could have purchased an army with it.
Some say the Doom occurred when the warring political Valyrian families finally killed too many of each other's fire mages and there were not enough left to keep the containment magic of the 14 Fires, the volcanoes where the dragons where discovered, going. With no containment magic, the volcanoes were prone to erupt.
Everyone always thinks the Faceless Men, an order of assassins from Braavos, (a city in northwest Essos founded by a mutiny of Valyrian slaves) caused the Doom but they have no idea how. But this guy on Reddit, and thus I, do know.
The family who sold Brightroar to the Lannisters used that gold to hire the Faceless Men and unleash them upon their rival families. Most specifically they had them assassinate the mages of the rival families in exchange for enough gold to field an army. Maybe they thought it would leave them as the only ones with the magic and power. Whatever they thought, without the mages the 14 fires were no longer stable.
So, Valyria goes BOOM.
Thus, the Lannister gold paid to the Valyrians for Brightroar was used to get the Faceless Men to kill rival families' mages, causing the 14 Fires to become unstable. The 14 Fires then erupt, the city of Valyria is destroyed, and the Valyrian Freehold collapses utterly. A chain reaction, it seems. Started by whom?
House Lannister.
First Lann the Clever swindles Casterly Rock from House Casterly, then his descendants destroy Valyria and fulfill the prophecy, and then their descendants, specifically Tywin and Cersei Lannister, manipulate their way to the Iron Throne.
Those guys are not to be messed with.