The Citadel is an archive of information for George R.R. Martin's A Song of Ice and Fire.
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Evidence revealed in A Storm of Swords points squarely at Joffrey as the instigator. At the queen’s breakfast he revealed, "... I am no stranger to Valyrian steel." (III: 663) Tyrion, suspicious of that statement, suggests that he will give a Valyrian steel dagger with a dragonbone hilt as a gift to Joffrey. Joffrey’s reply is awkward: "Joff gave him a sharp look. ‘You ... yes, a dagger to match my sword, good.’ He nodded. ‘A ... a gold hilt with rubies in it. Dragonbone is too plain.’" (III: 664) Finally, Jaime questions Cersei about the attempt on Bran. She reveals that Robert said in Joffrey’s hearing that it’d be a kindness to end the boy’s misery, but that they were all too weak. Jaime concludes that Joffrey, eager for his father’s attention, may have decided to prove that he was capable of doing it (III: 823).
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