Cersei Lannister sends her regards.
If you're all caught up on Game of Thrones Season 8, check out what happened to Ghost and Jon's dragon Rhaegal, all the extra meaning behind Missandei saying "dracarys," how there might be more dragons in Westeros, why the Battle of Winterfell's deaths were disappointing, what Melisandre said to Arya back in Season 3, and just what the battle tactics of Episode 3 actually were. You might also be wondering and , not to mention what new theories we have based on Episode 4.
The final season of Game of Thrones keeps seeing the culmination of plot threads that have been building throughout the course of the series: the battle between the living and the Night King, character development (and romances) building over years, and family relationships strained by new pressures. Now that the Battle of Winterfell is over, the backstabbing is beginning again in earnest on Game of Thrones.
The living might have defeated the Night King and stopped the White Walker threat, but there's another adversary still out there, potentially even deadlier: Cersei Lannister. And while her brother Jaime left her to fight the White Walkers and save the realm, Cersei is not the forgiving type. Back in the premiere episode, "Winterfell," Cersei set into motion a plan to exact revenge on Jaime and their brother, Tyrion Lannister--one that encountered a new twist in Episode 4, "The Last of the Starks." Bronn showed up in Winterfell and nearly carried out an order he got form Cersei in the premiere episode, to kill Tyrion and Jaime using a very significant crossbow.
It's the same crossbow Tyrion wielded way back in Season 4, and used to kill his father, Tywin Lannister. That was after Tyrion was tried and convicted of poisoning King Joffrey, and Tywin sentenced Tyrion to death, even though he was innocent. Jaime helped Tyrion escape, but before he did, Tyrion went to Tywin's room. There, Tyrion found Shae, his former lover who testified against him, and killed her. After that, Tyrion found Tywin on the toilet, and finished him off with a crossbow decorated with gold.
Cersei sending Bronn with that same crossbow is a reminder of Tyrion's betrayal of the Lannister family. She blames Tyrion for the deaths of her children--since Joffrey's death, she's maintained that Tyrion was responsible, but even learning from Jaime that Lady Olenna Tyrell (Diana Rigg) poisoned her son hasn't changed her mind. She also blames him for the deaths of Myrcella (Nell Tiger Free) and Tommen--as Cersei said in Season 7, when Tyrion killed Tywin, it left the Lannisters exposed.
"Do you have any idea what you did when you fired that crossbow?" Cersei asks Tyrion. "You left us open. You laid us bare for the vultures and the vultures came and tore us apart. You may not have killed Joffrey, but you killed Myrcella, you killed Tommen. No one would have touched them if Father was here--no one would have dared."
Sending Bronn with that the same crossbow Tyrion used to kill Tywin is exactly the kind of poetic message-sending Cersei loves. Like playing "The Rains of Castamere," it's a power move that builds her reputation, shows her spite for her enemies, and strikes fear in others. Lucky for Tyrion and Jaime, Bronn wasn't willing to take the two men out without talking through it, and they offered him Highgarden to stay the crossbow's bolt. It seems like we've seen pretty much the last of Bronn at this point--he might turn up in the end to claim his reward from Tyrion, or he might get a different kind of reward for double-crossing Cersei. There's still a chance for that crossbow to show up again to send another meaning-filled bolt into somebody's chest.