Most dragons, and Valyrians, perished during the Doom of Valyria. Those that survived were those residing on Dragonstone: House Targaryen. Originally a minor house of the Valyrian Freehold, the Targaryens suddenly found themselves the only survivors of their race and masters of the three remaining dragons:
- Balerion the Black Dread
The last dragon of House Targaryen (and supposedly in the world) died approximately 150 years before the events of A Song of Ice and Fire. Most of the dragons died in combat (with the majority coming from the Targaryen civil war called the Dance of Dragons). Each successive generation of dragons was smaller, sicklier, and shorter-lived than the previous.
It is believed that the deaths of the dragons coincided with magic leaving the world. There is also evidence that (AFFC spoiler)
was behind the death and demise of the last dragons.