Joffrey says that Rhaenyra Targaryen was “murdered by her brother, or rather, his dragon,” and that “it ate her while her son watched.”

Context

He says this in Game of Thrones while speaking with Margaery Tyrell, and the line is a grim spoiler for Rhaenyra’s fate in House of the Dragon.

Exact line

“Rhaenyra Targaryen was murdered by her brother, or rather, his dragon. It ate her while her son watched. What’s left of her is buried in the crypts right down there.”

What it means

The scene is referring to the later Targaryen civil war, where Rhaenyra is ultimately killed by Aegon II’s dragon, Sunfyre.

TL;DR: Joffrey spoils Rhaenyra’s death by bluntly describing her being killed and eaten by a dragon.