Daenerys, her fleet of ships, and her armies have docked at Dragonstone, which means they're officially on Westerosi soil. Cersei Lannister's advisors are most concerned about Dany's three dragons, however, which pose the greatest threat of all. Qyburn assures them that they have a plan at the ready, but what solution does Cersei have for Daenerys' dragons on Game of Thrones? There is a weapon in the crypts that may do the trick.
At first, it seems as if Qyburn might be bluffing. After all, Cersei destroyed the Sept of Baelor, killed most of the ruling class of King's Landing in a wildfire blaze, and basically prompted the suicide of King Tommen; she can't really take any more hits and keep her grasp of the Iron Throne. If she admits she has no idea how to defeat the dragons, it may be enough to incite rebellion before Dany even arrives. But Qyburn later takes her down to the cellars of the Red Keep, where old dragon skulls from the Targaryen dynasty are kept. It's there that he shows her his — quite literal — secret weapon. According to him, the finest artisans and blacksmiths in King's Landing have been working on it, using the old dragon skulls as target practice.
He shows her what looks to be a super powerful ballista — basically, a giant medieval arrow launcher. As Qyburn demonstrates using the skull of one of the biggest dragons in the cellar, the ballista can shoot a spear with enough force to crack a dragon's skull. He relays to Cersei that there were reports of dragons being wounded in Meereen by spears. And if a dragon can be wounded, then a dragon can be killed, he said. Of course, finding a way to break through tough, scaly dragon flesh and pierce their thick, enormous skills is something that can't be tested until a dragon is in their midst, which is where Dany's advantage lies.
But even if the ballista doesn't work, there is another way to gain control of the dragons. Some fans believe Euron's gift to Cersei will be Dragonbinder, a dragonhorn which he had possession of at one point in the books. Legend has it that whomever possesses a dragonhorn can control dragons. If Cersei had it, she could simply blow into it to wrest control of Daenerys' dragons, and there would be nothing Dany could do about it, even if she is their mother. So all hope is not lost for Cersei. She may have a few tricks up her sleeve yet.