He is the Lord of Horn Hill and the head of House Tarly. He forced his son Samwell Tarly to join the Night's Watch under the threat of death.
Randyll Tarly is the Lord of Horn Hill and the head of House Tarly, a vassal family of House Tyrell of Highgarden. He is a fierce, ruthless warrior and a highly effective general. He is the father of Samwell Tarly and a younger son, Dickon, and at least one daughter Talla.
Along with his Liege Lord Mace Tyrell, Randyll fought on the Targaryen side in Robert's Rebellion. He commanded the royalist victory at the Battle of Ashford - the only defeat that Robert Baratheon ever suffered in the entire war.
Samwell recounts to Jon Snow that even though he was his father's eldest son and heir, he was disgusted with Samwell's "womanly" interest in books and learning and disdain for martial valor and combat. Soon before Sam turned eighteen, his father took him aside and bluntly told him that he was unworthy to inherit the Tarly lands and titles. Randyll ordered Sam to take the black and join the Night's Watch. Randyll told his son that if he did not, he would take him out on a hunt in the woods and kill him in such a way that it looked like an accident. Terrified, Sam did as he was bid and left Horn Hill for the Wall.
At Stannis Baratheon's war council, Davos Seaworth names Randyll Tarly amongst the many lords that have declared for Renly Baratheon.
Samwell mentions his father by name when explaining the concept of family names to Gilly. When she points out that Randyll is a handsome name, he asks her to please not give it to her newborn son.
Sam quips to Gilly that his father is not a very "literary man". Later, surprised that Samwell is Randyll's son, Stannis Baratheon recounts to Sam how Lord Randyll defeated Robert Baratheon in the Battle of Ashford — the only battle Robert ever lost, for which King Stannis considers Randyll a fine soldier.
Since the Citadel only allows men within its walls, Samwell decides to send Gilly and her baby Sam to live with his family at Horn Hill. While Sam admits that he does not get on well with his father Randyll, he reassures Gilly that his mother and sister are nice.
When Sam and Gilly arrive at Horn Hill, Randyll assumes she is a prostitute from Mole's Town, something that he manages to come to terms with. She is dressed in southern clothes, and she, Sam, Randyll, Dickon, Melessa, and Talla all have dinner together. At the table, Randyll repeatedly berates Sam: he insults his weight, tells him the Night's Watch failed to make a man out of him, and says he will never be a great warrior, or inherit Heartsbane, the Tarly family's ancestral Valyrian steel sword. Gilly rushes to defend him, telling Randyll that Sam killed a White Walker on his way down to Castle Black. By saying this, she lets slip that she is in fact a wildling, a group of people who Randyll apparently considers to be subhuman. He banishes Sam from Horn Hill, but allows Gilly and her baby to remain. However, Sam takes them both with him when he leaves, along with Heartsbane itself.
Randyll travels to King's Landing at the request on Cersei Lannister, the new Queen of the Seven Kingdoms. Cersei implores Randyll, and the other lords of the Reach to pledge fealty to House Lannister in order to defend Westeros from Daenerys Targaryen and her combined army of Dothraki and Unsullied.
Jaime then manages to get Randyll on his own, after a brief confusion of names with Dickon who is mistaken as "Rickon" to Jaime. He then makes it clear that Randyll is the most prominent of the lords of the Reach, and that the others will look to him when choosing sides. Despite Jaime mentioning the possibility of the Dothraki invading his lands, Randyll still announces that he doesn't wish to be dishonorable to Olenna. Jaime then promises to give him the title of Warden of the South, should they succeed in triumphing over Daenerys.
Despite his initial objections, Randyll eventually sides with the Lannisters and, together with Jaime Lannister and Bronn, leads the main part of the 10,000 men strong Lannister army against Highgarden where they easily take and loot the castle, killing Olenna Tyrell in the process.The prisoners of war are being herded by the Dothraki to Daenerys and Drogon behind her, waiting menacingly. Daenerys appeals to them, bringing up the rumours spread about her by Queen Cersei, and warnings of brutality that the dragon Queen would bring - burning down homes, and murdering families. She assures she is not here to murder, but to destroy the wheel of power that rolls over the rich and poor alike, to no one's benefit but people like Cersei Lannister. She offers them a choice - bend the knee and join her in her quest to make the world a better place than ever before, or refuse and die. Tyrion looks at her apprehensively upon hearing this, most of the soldiers kneel immediately, intimidated by Drogon. Most- except for Randyll Tarly, his son Dickon, and well as a handful of men. Daenerys summons Randyll forward, but his insists he already has a Queen. Tyrion recalls he didn't pledge to Cersei until recently, and that she murdered the rightfulQueen, destroying House Tyrell for good. He observes Randyll's allegiances are questionable to which Randyll answers there are no easy choices. He reminds Tyrion that Cersei is at least a true Westerosi, and that Tyrion is a kinslayer, having killed his father, as well as supporting a foreigner; bringing savages to their continent. Daenerys accepts his answer, and prepares to carry out a sentence, but Tyrion intervenes with the possibility of sending Randyll to The Wall, instead of death. Randyll affirms Daenerys cannot send him to the Wall as she is not his Queen.
Daenerys signals for three Dorthraki men to apprehend him, but Dickon suddenly speaks up insisting he will have to be killed too. Randyll, horrified, tells his son to shut up. Tyrion reminds Dickon that he is the future of House Tarly, and insists that he better bend the knee, reminding him how what happened to House Tyrell, which Randyll silently nods in agreement so his son may be spared. Dickon refuses to relent his decision, thereby Tyrion proposes to Daenerys about having them committed to the cells instead, but Daenerys, being ruthless but pragmatic, does not wish to grant the reputation of putting traitors in chains, or many would take advantage. She resumes to carry out the sentence for them both, as they hold hands. Drogon unleashes his dragonfire, roasting Randyll and Dickon alive and reducing them both to flame and ash in seconds, killing them instantly. Terrified, the remaining soldiers kneel as Tyrion reflects uneasily over the execution.
A legendary soldier, Randyll Tarly had no patience for his son Samwell’s studious nature. He forced Sam to join the Night’s Watch, making his second son Dickon heir to Horn Hill. When Samwell returned to his family’s castle with Gilly, Randyll was disgusted to learn she was a wildling and banished Sam for good. After being promised the title Warden of the South in exchange for his loyalty to Cersei, Randyll fought alongside Jaime Lannister’s army at Highgarden. He and Dickon were both incinerated by Drogon when they refused to submit to Daenerys Targaryen.