The Doom of Valyria was a great cataclysm that obliterated Valyria and its civilization in a single day and night. Yet, the causes of this destruction are not known even four centuries after it happened but there are several debated possibilities. But for RP purposes, I’m going to actually use almost all of them convoluted into one big chain of events that unleashed the ultimate demise of the Valyrian Freehold. 

Valyria was both a peninsula and the name of the capital city of the Valyrian Freehold ( an empire with no emperor ), built at the base of a chain of volcanic mountains that thrived there for five thousand years. For this feat to be possible, the Valyrians would need to control the fury of the Fourteen Flames in order to prevent them from erupting and destroying the city, which is something they do through the use of magic for five millenia. Prophecy, many would believe, is something magical in nature and this is something that the Valyrians believed in, given that one of the gifts of their blood endowed them with prophetic dreams. The Valyrian Freehold expanded through their power in dragons, magic and the use of slaves but the more power they possessed, the more turmoil was bred. 

Even though the Freehold had no emperor, it was ruled by forty noble families ( it’s not known if all were dragonlords or only half or some ), and within these families, a hierarchy was instituted. This naturally brings on a rivalry between the families that sought to raise their positions and those at the top that wanted to maintain it and dominate over those below. They employed different ways to achieve this, including deception and assassinations of members of those families, many of which could’ve been mages. Valyrians were already delving in dangerous and volatile arts for a long time, such as blood magic, so the deaths of these mages unsurprisingly had a domino effect; one that only amounted to the rest of the pieces that had been falling into place before. 

As prophecy foretold that gold from Casterly Rock would destroy them, they did not trade with the Kings of the Rock–But the Lannisters did get their hands on a valyrian steel sword in the century before the Doom. Prophecy as it so often happens, can be taken or manifested in different ways. The Targaryens knew how to interpret it in time to escape to the safety of Dragonstone twelve years before the Doom, something that was seen as cowardice by the other Dragonlords, since Targaryens were amongst the weakest of them. But maybe because of this view from below, Aenar Targaryen had a different perspective of the Freehold. It is possible that the Targaryens after moving to Dragonstone, were the ones to trade with Casterly Rock and sell to them the valyrian steel sword Brightroar, for an amount of coin that was enough to buy entire armies–Or maybe just enough to fund the service of Faceless Men. 

Some believe that the Faceless Men were ‘born’ in the mines of the Fourteen Flames, where slaves cried out for deliverance. It’s no coincidence then that the center of this guild is Braavos, a city built by escaped valyrian slaves, and that their purpose would be to abolish all such related valyrian slavery to which they were subjected to. Did perhaps Aenar Targaryen share this same ambition, that one would be seen reflected in his descendant centuries after ( Daenerys Targaryen ), seeking to end slavery? Regardless, the Faceless Men were present ( originated ) in Valyria before the Doom as so their own histories say, and they gave the ‘gift’ ( death ) to their valyrian masters. Was the Casterly Rock gold what funded the very first Faceless Men as they imparted the gift, and did it come from the Targaryens?

For five thousand years, the Valyrian Freehold has been dominating most of the known world thanks to slavery and dragons, creatures that were said to number in the several hundreds or thousands, weapons of war and destruction. As they had so demonstrated, their power was unrivaled and it would end in annihilation to those who dared challenge them ( Ghis and the Rhoynar ). The only way that such an all powerful civilization could end would be from within, by the fire and blood on which they thrived in the first place. The inner struggle and the assassinations from both Valyrians and Faceless Men of mages that used their magic to keep the volcanoes from erupting, was most likely what destabilized the so careful magical balance that tamed the Fourteen Flames. 

Perhaps in one last attempt to maintain that control, remaining mages and dragonlords alike gathered in Valyria to concoct a great spell of power. Maybe the hellhorns that sounded in the Doom were meant to play a part in ending the menace, only for it to go horribly wrong and usher forth instead their own destruction.

This is always still a work in progress, to be developed when/if we are given more information in books and/or shows.