- "Lyonel Baratheon, black of hair."
- ―Excerpt from The Lineages and Histories of the Great Houses of the Seven Kingdoms
Lyonel died of a fever at the age of 45, leaving Storm's End to be inherited by his only son.
He has an entry in the The Lineages and Histories of the Great Houses of the Seven Kingdoms, as read by Eddard Stark.
Behind the scenes
Lyonel Baratheon's entry in the prop for The Lineages and Histories of the Great Houses of the Seven Kingdoms was written during production of the first season of Game of Thrones, which was released in 2011. His heir, Ormund Baratheon, was first introduced in the book series with The World of Ice and Fire in 2014. The Season 1 prop establishes Lyonel Baratheon as the husband of Rhaelle Targaryen and father of Steffon Baratheon, but subsequent Histories & Lore featurettes established Ormund as Steffon's father. With the effective retcon of Steffon's father, it is unclear if Rhaelle Targaryen married Ormund as in the book series (as she is listed in the prop as Steffon's father) or Lyonel (her husband per the prop). Lyonel has birth and death dates in the prop; 204 and 249 AC, respectively, but these have to be considered retconned as Steffon's birth year per the prop, 229 AC, becomes impossible if he were to be the grandson of a man born in 204 AC.
In the books
Lyonel Baratheon appears in The Hedge Knight, one of the stories in the Tales of Dunk and Egg series, which focuses on Ser Duncan the Tall and Aegon V Targaryen during Aegon's time as Ser Duncan's squire. Ser Lyonel is known as "the Laughing Storm". He would often break into booming laughter the moment tourney rivals touched his shield, and laugh all the time he was mounting and charging and knocking them from their stirrups. If his challengers wore any sort of crest on their helm, Ser Lyonel would strike it off and fling it into the crowd.
Lyonel participated in the tourney at Ashford Meadow, challenging Ser Robert Ashford in the first tilt. Breaking nine lances, both lost their saddles on the tenth course and fought onwards on foot, sword against mace. The battered Robert eventually admitted defeat, and Lyonel became champion. Before fighting for Ser Duncan the Tall, he knighted Raymun Fossoway.
Because Lyonel was a faithful supporter of Aegon V as the Lord of Storm's End, Aegon was pleased to arrange the betrothal of his eldest son and heir, Prince Duncan, to Lyonel's daughter. However, Prince Duncan broke the betrothal when he fell in love with and married the commoner Jenny of Oldstones. The outraged Lyonel felt that the honor of House Baratheon had been insulted, and rose in rebellion against the Iron Throne, declaring himself Storm King. A short rebellion followed, but this was ended when Ser Duncan the Tall, now Lord Commander of the Kingsguard, defeated Lyonel in a trial by combat. Prince Duncan renounced his claim to the throne, and Aegon agreed to betroth his youngest daughter, Princess Rhaelle, to Lyonel's heir, Ormund Baratheon. Rhaelle was sent to Storm's End to serve as Lyonel's cupbearer and companion to his wife. In the TV series continuity, Rhaelle married Lyonel rather than Ormund and her relation to Aegon V is not clear.
The entry for Lyonel in the Lineages book from Season 1 (see this image) says that he died at the age of 45, but this is unlikely to be true in the book continuity, and possibly a case of the TV writers just filling in the gaps of knowledge for the prop book. In the novels, he apparently died between 239 AC and 245 AC, and when he took part in Duncan's trial by seven in 209 AC he was already a large and powerful warrior. Assuming that Lyonel was at the very least 16 years old in 209 AC (like Duncan was, though he was probably older), he would have been born around 193 AC. Lyonel therefore could have been at most around 52 years old when he died - but from how he is presented in The Hedge Knight, he was apparently somewhat older than Duncan. More information may be filled in with future Tales of Dunk & Egg novellas.