Aerion was born in Tyrosh, the son of a prostitute. On her deathbed, his mother claimed that he was descended from the long extinct Blackfyre line. After her supposed death, his Golden Company comrade Varyo Velaryon presented him with a set of shards, purported to be the remains of the lost blade Blackfyre.
Varyo and Robert Manderly used him to create a possible campaign against Westeros. At some time between the 'death' of his mother and his crowning, Aerion discovered that he was not a real Blackfyre, and that it had been a ploy.
Aerion and Varyo Velaryon spearheaded the Council on Bloodstone in an attempt to rally support behind Aerion's claim to the Iron Throne. He garnered the backing of a handful of lords including Loren Lannister, the Warden of the West. Aerion then returned with Loren to Casterly Rock where he was wed to Loren's daughter, Ashara Lannister.
With King Harys Baratheon and his troops in the south, House Lannister and the Golden Company moved to take King's Landing. With help from allies inside the city, they were able to open the Mud Gate and the Lion's Gate, and subsequently sack the capital.
Aerion, however, only sat the Iron Throne for a moment before being killed by Varyo Velaryon. The plan had been a ruse put together by Varyo and Loren Lannister in order to seize the throne for Loren's son, Damon Lannister.
"You would do well to remember that the Dragon bow's to none."
"A false dragon, something you would do well to remember." - An exchange between Aerion Blackfyre and Varyo Velaryon