Lord Tywin marches from Harrenhal, and Arya considers sneaking away with them, but Weese threatens to hand any runaway over to Vargo Hoat to be maimed (not a figure of speech here), and this scares the prospect out of her.
Arya looks for Jaqen H'ghar to whisper another name before everyone leaves, but Weese sends her to the armory to fetch a sword for Ser Lyonel.
When she returns with the sword, Weese slaps her for being so slow and sends her on another task, promising to beat her if she isn't quicker.
Arya goes looking for Jaqen first and whispers Weese's name in his ear.
The next day, while watching Lord Tywin, the Mountain, and the others ride out for the battlefield, Arya is struck with the realization that she could have ended the war had she only whispered Tywin's name.
She runs to find Jaqen and ask for a redo, but she finds a crowd standing over Weese's dead body instead.
She spies Jaqen in the crowd, and he holds two fingers casually against his cheek.