For thousands of years the Valyrian Freehold was the heart of the Known World; it spanned most of Essos, and was an advanced civilisation known for both its culture and its military power. It was a land where both magic and dragons thrived, and then suddenly it was destroyed in what's now known as the Doom of Valyria.
In 114BC, every hill for 500 miles exploded; the Fourteen Flames - the volcanoes that stood in the Freehold - poured lava and ash over the Earth; everyone, even dragons, were killed by the smoke, ash, and fires that filled the air; the Valyrian Peninsula was broken, creating the Smoking Sea; and Valyria was no more.
A whole culture wiped out, from people to spells, knowledge to dragons, and yet no one knows exactly what happened. Some say it was the Fourteen Flames, and a natural disaster. Others attribute it to the gods or dragons; magic or ancient curses - and the truth is likely a mix of all those and more.