Ardyn Martell, also called Blackfang, is a recurring character in the second, third, fourth, fifth, sixth, and seventh seasons. He is an exiled member of House Martell of Dorne, and is the son of Lewyn Martell. He is the personal bodyguard, assassin, and enfocer of Lord Petyr "Littlefinger" Baelish.
Little is know about Ardyn's past, nor is it known how he came to be in the service of Lord Baelish. The rest of House Martell holds him in great contempt; likewise, he harbors hatred towards them as well, particularly his father and his cousin, Doran, both of whom he considered "weak and foolish".
The animosity between Ardyn and the rest of House Martell is due to the fact that, during Robert's Rebellion, it was rumored that he defected from Dorne to Tywin Lannister as part of a plot for Tywin to gain control of Dorne. It is also alleged that he fought in the Sack of King's Landing and witnessed the brutal rape and murder of his cousin Elia by Ser Gregor Clegane, and, according to Oberyn Martell, simply "stood there and smiled" as it happened, despite Elia's pleas to help her. None of this was ever proven to be correct, but both Doran and Oberyn believed it to be true.
Ardyn was a participant in King Joffrey's nameday tourney and was supposed to fight Ser Dontos Hollard, which ultimately did not happen due to the fact that Ser Dontos showed up late to the tourney and was in no condition to fight due to being drunk.
Though Ardyn's supposed betrayal of the Martells during Robert's Rebellion was never proven, during Lord Baelish's trial at Winterfell, it was proven to have indeed happened when Bran Stark reveals that he'd used his greensight to prove it, though Ardyn vehemently denies it. When Yohn Royce refuses his request to escort him back to the Vale, Ardyn says that he didn't have the authority to speak to his lord that way, to which Royce coldly replies that he was a "traitor" and no longer had any authority.
Ardyn is visibly shocked when Lord Baelish starts begging Sansa Stark for his life. When Arya Stark slashes Lord Baelish's throat, he calls her a "murderous little cunt" and advances on her with his own sword, but before he can harm her, Aenir Targaryen comes up behind him and stabs him through the back.