Maekar Targaryen was the son of King Aegon VI Targaryen, born to him and his wife the Queen Sansa Stark in the year 310AC. He was granted the title Lord of Harrenhal following the extinction of House Strickland in the War of the Lords Scorned. He is the founder of House Targaryen of Harrenhal.
Following the War of the Lords Scorned, Maekar was awarded Harrenhal and immedietly went to work attempting to restore the castle. To do so, he enlisted the help of the Smallfolk and employed much of his and their wealth to finance the project. His actions would not be well received and the Smallfolk silently held him in contempt for years. Ultimately, The Red Brotherhood would be formed near Harrenhal and would be responsible for banditry near Harrenhal for years.
Maekar initially paid no notice to the bandits, deeming them beneath him and not worth his time. Maekar, serving as Master of Coin upon his brother Aemon Targaryen's council was frequently travelling between Harrenhal and King's Landing based on his duties as a member of the Small Council. One fateful day, Maekar would be kidnapped by the Red Brotherhood and brought to Rushing Falls before their leader Hagrim the Black. Initially believing that he would be freed after a ransom, Maekar is initially confident and even taunts the bandit.
The choice would end up being a fatal one, as Maekar was tortured and butchered by Hagrim immedietly. With Maekar's death, his brother Aemon Targaryen and his son Maegor Targaryen rode against the Red Brotherhood and put them to the sword, earning vengeance with fire and blood.