House Targaryen is famous for its so-called Madness, a genetic feature supposedly derived from centuries of continuous incest and inbreeding. The Targaryens rarely married outside their family so as to maintain the purity of their bloodlines, inadvertently worsening the mental stability of each successive generation. Maester Aemon's brother Aerion Brightflame consumed wildfire to "become a dragon."
Similarly, Aerys II Targaryen planned to burn an entire city to the ground so he could arise from the ashes, like some kind of macabre phoenix. It's likely that Daenerys also suffered from this incurable malady, especially given her recklessness and cruelty during the Battle of King's Landing. The Targaryens in House of the Dragon are yet to demonstrate conclusive symptoms of Madness, but some of them already seem halfway there.
Warning: Spoilers for House of the Dragon below
7 King Jaehaerys Targaryen Was Probably The Least Mad Member Of His Family
Jaehaerys Targaryen reigned over Westeros for fifty-five years, climbing the Iron Throne at the young age of fourteen. Known for his peaceful and amiable nature, this King is widely regarded as the greatest Targaryen ruler in history. Grand Maester Benifer claimed that Jaehaerys was "learned as a maester and pious as a septon," whereas Queen Alyssa called him "the best of [her] three sons."
King Jaehaerys treated everyone around him with dignity and respect, from lowborn peasants to landed gentry, earning him love and admiration across the realm. Unlike most of his ancestors (and descendants), Jaeherys understood the value of knowledge, spending the bulk of his time immersed in the royal library's tomes and scrolls. It's safe to say that Jaehaerys was the least Mad member of the Targaryen dynasty.
6 Queen Aemma Doesn't Exhibit Any Targaryen-Specific Afflictions
Although technically born into House Arryn, Aemma is half-Targaryen. Her mother, Princess Daella, was one of King Jaehaerys' thirteen children, making her Viserys' first cousin. Aemma becomes the Queen after Viserys is selected to succeed their grandfather, creating a legacy she never wanted in the first place. And yet, she dutifully fulfills her obligation to the Iron Throne without any complaints.
Aemma undergoes several miscarriages during her marriage, leaving her feeling exhausted and miserable. She finally informs her husband of her intentions, stating that she's "mourned all the dead children [she] can." Viserys accepts her decision, but only because he's confident that her final child will be the male heir he had been eagerly praying for. Sadly for Viserys and Rhaenyra, Aemma dies giving birth to Baelon, who promptly follows his mother to the afterlife. It might be the Arryn blood in her veins, but Queen Aemma doesn't exhibit any Targaryen-specific afflictions.
5 King Viserys I May Have Succumbed To Targaryen Madness If He Had Lived Long Enough
The son of Prince Baelon and Princess Alyssa Targaryen, Viserys was third in the line of succession to the Iron Throne at the time of his birth. Following the deaths of his father and uncle, Viserys quickly gained the attention of Westerosi nobles as a potential successor. The Great Council inevitably chooses him over his cousin, Rhaenys, crowning him the fifth Targaryen King of Westeros.
Viserys inherits a relatively stable administration from his grandfather, explaining why he rules the realm with a gentle hand. His brother considers his brother too weak-minded to be a capable ruler, and not without reason. Despite his unquestionable largesse, Viserys' noncommital approach to governance would indirectly spark the so-called Dance of the Dragons. It can be argued that Viserys would have eventually become a victim of Targaryen Madness if he had lived long enough.
4 Princess Rhaenys Targaryen Seethes With Bitterness And Hatred, Even If She Rarely Displays Her True Feelings
Born to Lady Jocelyn Baratheon and Aemon Targaryen, the Prince of Dragonstone, Rhaenys was all set to inherit the Iron Throne after her grandfather and father. Her grandmother Queen Alysanne even referred to her as "our Queen to be." However, Aemon's accidental death prompts King Jaehaerys to name his younger son, Baelon, as his heir.
This creates friction within House Targaryen, one that is finally mitigated by the Great Council's male-centric decision. Rhaenys was renowned for her great beauty as well as her dragonriding skills, but years of being sidelined have played havoc with her capacity for patience. Now known as the "Queen Who Never Was," Rhaenys Targaryen seethes with bitterness and hatred, even if she rarely displays her true feelings. Rhaenys would serve on Queen Rhaenyra's Black Council until her death.
3 Princess Rhaenyra Transforms Into A Power-Hungry Tyrant In The Books
Rhaenyra Targaryen is currently a fan-favorite, although she's soon going to lose that distinction if her path overlaps with the book version of her character. As the heir to the Iron Throne, Rhaenyra fully intends to carry her father's legacy forward, even if it means destroying her enemies. House Targaryen splits down the middle after Viserys' death, with an equal number of factions supporting both Aegon and Rhaenyra's claim to the Iron Throne.
This conflict soon turns into a raging inferno, destabilizing Westeros on multiple levels. Rhaenyra loses her youthful optimism over the course of the Targaryen Civil War and transforms into a power-hungry tyrant, much like her descendant Daenerys. Aegon ultimately has his half-sister executed by feeding her to his dragon, Sunfyre.
2 Aegon II Leaves House Targaryen Fractured And In Near-Ruins
The son of Alicent Hightower and Viserys Targaryen, Prince Aegon is far too young to actually show any signs of Targaryen Madness. In fact, he doesn't even have a speaking role at this point in House of the Dragon. The book version of this character, however, paints a radically different picture. Aegon inherits the Iron Throne from his father before engaging in an all-out war with his half-sister.
Rhaenyra demolishes his armies with relative ease, leaving him more resentful than ever. Aegon begins self-medicating with strongwine, a particularly potent form of alcohol, which invariably aggravates his already fraying mental condition. Unfortunately, Aegon's temperament fails to improve even after he defeats and kills Rhaenyra. The King's death leaves House Targaryen fractured and in near-ruins, explaining why future scholars would eventually list him among the worst rulers of all time.
1 Prince Daemon's Erratic Behavior And Lack Of Conscience Make Him The Worst Imaginable Heir To The Iron Throne
Prince Daemon Targaryen desires the Iron Throne more than anything else in the world, which is why he foolishly attempts to woo his niece. He strongly believes that he would be a better ruler than his brother, an unsubstantiated claim based on his bloated ego. If that's not Mad enough, Daemon "dissolves" his marriage to Rhea Royce by beating her to death with a rock and later scorns his wife's cousin when he accuses the Prince of murder.
Despite his combat prowess and martial leadership skills, his erratic behavior and lack of conscience make him the worst possible choice to succeed King Viserys. Furthermore, Daemon's violent streak tends to overshadow both his common sense and his duty to the realm. Daemon eventually marries Rhaenyra and sires three children with her, two of whom would go on to sit on the Iron Throne.