Matt Smith's Prince Daemon Targaryen wields the famous and deadly longsword - Dark Sister in House of the Dragon. This Valyrian steel weapon and Blackfyre are the two famous ancestral swords of House Targaryen. While Blackfyre was Aegon the Conqueror's sword, his older sister and wife, Visenya Targaryen wielded the slender blade known as Dark Sister. King Aegon I Targaryen used Blackfyre during the invasion of the Iron Islands to slay his enemy, the Lord of Volmark - Qhorin Volmark. He used it during the First Dornish War as well.
Despite Aegon's swordsmanship, Visenya felt his guards weren't enough to protect him from the perils of sitting in the most dangerous seat in the Seven Kingdoms - the Iron Throne. After a failed assassination attempt on Aegon and Visenya's lives in 10 AC, the latter demonstrated the danger the Dornish War had placed them in with the help of her longsword. She managed to slash Aegon's cheek before any of his guards had the chance to respond, and following this, he entrusted her with the responsibility of forming the Order of the Kingsguard. Aegon the Conqueror had two sons from his two Queens. While Rhaenys' Aenys was the eldest and thus the heir to the throne, Visenya's Maegor was the younger one. Visenya presented her Valyrian steel longsword to Maegor in 25 AC. Dark Sister changed hands many times over the years and per HBO's House of the Dragon, it is in Daemon Targaryen's possession.
From Maegor The Cruel To Daemon Targaryen
Maegor, also known as Maegor the Cruel wielded Dark Sister at the age of thirteen. Game of Thrones lore states that upon King Aenys' ascension to the Iron Throne in 37 AC, he bestowed their father, Aegon's Blackfyre longsword to his younger brother, and the more-skilled warrior, Prince Maegor. From then on, Maegor hung the Dark Sister in his chambers at Dragonstone and wielded Blackfyre.
House Targaryen's history reeks of betrayals and Maegor usurping the throne of Aenys' children is the earliest example of a blatant power grab in the house of the dragon. Maegor's mother, Visenya held Aenys' wife, the Dowager Queen Alyssa Velaryon, and her youngest children, Jaehaerys and Alysanne as her prisoners on Dragonstone. However, after Visenya's death in 44 AC, Alyssa, and her children escaped, and they stole Dark Sister from Visenya's chambers.
Dark Sister was wielded by Jaehaerys the Wise in 48 AC when he laid his claim to the Iron Throne. After Maegor's tyrannical reign ended, Jaehaerys became the fourth king on the Iron Throne. Dark Sister was also used by Jaehaerys' son, Prince Baelon Targaryen aka Baelon the Brave in 92 AC. He used the longsword to slay Myrmen, thereby avenging the death of his brother, Prince Aemon. As for Prince Daemon Targaryen, his grandsire, Jaehaerys I bestowed Dark Sister upon him, as he was a skilled fighter.
Dark Sister In House Of The Dragon
Blackfyre is wielded by Viserys I and afterward by Aegon II. The Rogue Prince, Daemon carries Dark Sister. He wields the longsword at the Heir's Tournament, held in honor of the upcoming birth of Viserys' heir. In House of the Dragon season 1, episode 1 "The Heirs of the Dragon," Prince Daemon jousts against Ser Criston Cole and is unhorsed. In an arrogant display, he wishes to continue and draws Dark Sister on Ser Criston. The haughty Daemon prematurely celebrates his win but Ser Criston strikes him from behind, knocking him over. This leaves Daemon no choice but to yield to a man inferior in rank and that too from a minor house.
Not only does Daemon lose the joust, but he also loses the contest of arms despite wielding the much superior Valyrian steel longsword. Daemon also wields Dark Sister while fighting the Triarchy for the Stepstones. He uses his intricate longsword to slay a man named Craghas Drahar, a self-styled Prince Admiral of this Triarchy.
The second son of King Viserys II, Prince Aemon aka the Dragonknight was the next Targaryen to own Dark Sister. Histories remember him as the noblest knight and a legendary swordsman. He was the Lord Commander of King Aegon IV's (Aegon the Unworthy) Kingsguard. Prince Aemon was known for his jousting, valor, and bravery. After him, Dark Sister was presented to Ser Brynden Rivers, also known as, Lord Bloodraven. He was Aegon the Unworthy's illegitimate son from his sixth mistress, Lady Melissa Blackwood.
Lord Bloodraven took the Black and went on to become Lord Commander of the Night's Watch in 239 AC. He disappeared while ranging beyond the Wall and as for Dark Sister, the longsword's whereabouts remain unknown in the lore. Ned Stark's youngest daughter, Arya mentions Dark Sister while serving as Lord Tywin Lannister in Game of Thrones season 2. "Visenya Targaryen was a great warrior. She had a Valyrian steel sword she called Dark Sister," she tells Lord Tywin. As far as Daemon Targaryen is concerned, the upcoming civil war in House of the Dragon will provide him with plenty of opportunities to wield Dark Sister.