Maelys Blackfyre

"All the same, his descendants continued to threaten the Targaryens until half a century later, when Barristan the Bold slew Maelys the Monstrous on the Stepstones, extinguishing Daemon's line."

- Varys

Maelys Blackfyre, better known as Maelys the Monstrous, is an unseen character in Game of Thrones. He is not expected to appear in the series, being long dead by the time it begins.

Background
Maelys Blackfyre was the last of the Blackfyre Pretenders to the Iron Throne. Like previous members of House Blackfyre, he proclaimed himself the rightful king as a descendant of Daemon Blackfyre, bastard son of King Aegon IV Targaryen, while the main Targaryen line descended from Aegon IV's lawful son King Daeron II Targaryen. Maelys rallied an army of mercenaries in the Free Cities and began a conquest of the Stepstones as a prelude to invading Westeros itself. The Targaryens attacked the Stepstones before he could launch the invasion, leading to the War of the Ninepenny Kings. This conflict ended when Maelys was killed in the Stepstones by the young Ser Barristan Selmy, extinguishing Daemon Blackfyre's line and ending further threat of Blackfyre pretenders.

In the books
In the A Song of Ice and Fire novels, Maelys Blackfyre was the last male descendant Daemon Blackfyre. He had a huge upper body and a second head sprouting from his neck. This was said to be the head of his twin whom he had absorbed in the womb, earning him the nickname "Maelys the Monstrous", as he had become a kinslayer, though not by choice.

Maelys was named Captain-General of the Golden Company after killing his cousin Daemon when they fought for the position, becoming a kinslayer for the second time, although this time by choice. During the War of the Ninepenny Kings, Maelys was one of the Band of Nine, a group of merchants, sellswords, and pirates who joined forces to carve kingdoms out for themselves. After conquering the Disputed Lands, Tyrosh and the Stepstones, Maelys convinced the other members of the band to attack Westeros and aid him in retaking what he claimed was his birthright. However, the Targaryens acted swiftly and sent their own force out to the Stepstones to meet him. There, Maelys was slain by Ser Barristan Selmy in single combat on the Stepstones, ending the Blackfyre pretensions to the Iron Throne.

The exact line of descent from Daemon I to Maelys is unclear. Daemon had seven sons and several daughters. Based on chronology, he was apparently Daemon I's grandson, but it isn't known from which of his children.