One Piece chapter 1166 reveals a crucial detail about Blackbeard and Shanks. This chapter confirms that Figarland Garling (Shanks’ father) was the one who delivered the final blow to Rocks D. Xebec. Since Shanks is Garling’s son and Blackbeard is a direct descendant of Xebec, the reason behind their tension is now clear.
It’s evident that Blackbeard knows about his father’s death. Being a child during the God Valley incident, he might not have understood that Xebec himself sought liberation. As a result, Blackbeard now harbors a grudge against Shanks, the descendant of the man who killed his father.
One Piece Chapter 1166: The Eternal Feud Between Shanks and Blackbeard

The information from chapter 1166 proves that the hostility between Shanks and Blackbeard is not random but rooted in a deep cause. Blackbeard likely resents both Figarland Garling and Shanks for taking away his father when he was young.
Shanks and Blackbeard have always had a challenging relationship. They first met when the Roger Pirates clashed with the Whitebeard Pirates. Later, they crossed paths again and fought at some point. Shanks even admitted to Whitebeard that the scar on his face was caused by Marshall D. Teach, and it’s the only injury that ever truly bothered him.
Even at Marineford, tension between Shanks and Teach remained, with Shanks being the only one aware that Blackbeard was a massive threat. Meanwhile, Blackbeard declared it wasn’t yet time to fight Shanks. More recently, Shanks has been closely monitoring Teach, gathering intelligence and watching every move of the pirate.
Will Blackbeard Face Shanks Instead of Luffy?

During the God Valley incident, Blackbeard became an orphan. Meanwhile, the son of the man who killed his father, Shanks, lived without hardship. Not only was Figarland Garling still alive, but Shanks was also adopted by Gol D. Roger into his pirate crew.
Needless to say, Blackbeard has many reasons to oppose Shanks. His grudge will likely lead to a final battle between the two. It’s possible Shanks will take on Blackbeard directly, leaving Luffy to focus on the greater threat of Imu.
Blackbeard caused a scar that Shanks will never forget. Because of this, Shanks won’t find peace until the score is settled. With both having valid reasons to fight, it seems their clash is destined.