- "My new blade. 'Hearteater' I've named it."
- ―King Joffrey Baratheon
Before leaving the Red Keep to oversee King's Landing's defenses against Stannis Baratheon's attack, King Joffrey orders Sansa Stark to see him off with a kiss. He then draws Hearteater and commands her to kiss it. He mentions that after the battle he will have her kiss it again and taste his uncle's blood. Sansa asked if that means he will be fighting the vanguard, as Robb Stark does, even though to Joffrey he isn't a real king. Angered, Joffrey says that someday she can lick Robb's blood off Hearteater as well. Ultimately, Joffrey never wields the sword in battle, but on the advice of his mother retreats back to the Red Keep when it looks like the city's outer walls will be breached.
In the books
In the A Song of Ice and Fire novels, Joffrey has a habit of frequently getting new and quite impressively made swords, and giving them fierce names like "Lion's Tooth" or "Hearteater", but he never actually swings a sword against an opponent at any time in the narrative. When he does personally execute people he uses a crossbow, because he doesn't actually know how to wield a sword.
Joffrey receives a new Valyrian steel sword, Widow's Wail, from his grandfather Tywin on the day of his wedding to Margaery Tyrell, replacing Hearteater - for a few hours, until Joffrey is poisoned at his own wedding feast later in the day. Many lords own more than one sword, but usually only one is "the" single, ancestral sword of their family. As a Valyrian steel sword, Widow's Wail would replace Hearteater as the sword passed down to the head of House Baratheon of King's Landing - and Martin has confirmed that ownership of Widow's Wail technically passed to Joffrey's younger brother Tommen after his death. No mention is made of what happened to Hearteater after Joffrey received Widow's Wail, or of what he intended to do with it.