Arya parted amicably from the Hound, after sneaking him into the Keep. As amicably as possible for the two of them, at least. "I hope you find your unfinished business," she had said.
"Fuck off," he had replied, but with an affectionate smile. And they had parted.
Arya slipped from shadow to shadow, all the way to the throne room. It wasn't hard to stay hidden. The soldiers were out on the walls manning the Scorpions, or on the fields clashing with the Targaryen and Northern forces. The servants were huddled in their quarters, leaving the halls comfortingly quiet. Two guards stood posted outside the throne room's doors. Arya regarded them cautiously, plotting how to kill them both without alerting Cersei inside.
Arya tensed as she heard a creak. She watched as a small section of solid wall slid back to reveal a small passageway. Jaime Lannister cautiously appeared from the passageway. Arya reached for her dagger. Has he come to fight for Cersei? She relaxed her grip as she saw Brienne and Podrick behind him.
Arya grimaced as the guards spotted them and shouted out an alarm. Stupid, impulsive Jaime, she thought. In seconds, Jaime had jumped out and killed both guards. She watched them count to three, then rush into the throne room. Arya crept closer. She could hear voices, but couldn't make out the words. She crept up to the door.
"Take my brother alive, but kill the other two," she heard Cersei's command. Then the boots of half a dozen or so queensguard, followed by heavy, thudding footsteps that could only be the Mountain. Arya drew her dagger, prepared to leap into the fight.
Footsteps were echoing down the hallway. Arya quickly hid behind a column and held her breath. She looked up to see the Hound approach, then disappear into the throne room. "Brother, time to put you out of your misery," she heard his voice say darkly.
Suddenly, an explosion of clanging and grunting erupted from the room. Arya leaped through the door with her dagger drawn. She glanced quickly over the battle. The Hound and the Mountain were furiously clashing, oblivious to the fighting around them. Cersei was slipping away through a door in the back of the room, with Jaime in pursuit. The last name on my list. Arya ran to follow them, when -
"AAURRGHHH!" The Hound's scream echoed around the room.
Despite herself, Arya glanced over her shoulder to see him lying on his back with blood pooling around him. The Mountain was nearly on top of him, pushing down with his sword in both hands. The Hound was pushing back with all his strength as the Mountain, sickly green and emotionless, bore down with all his weight. His sword was inches from the Hound's face and getting closer.
Cersei's robes swished through the doorway and out of sight. Cersei, Cersei, Cersei. The last name on her list. The name that she had repeated thousands of times. The hated mastermind who had destroyed Arya's family and caused the slaughter of so many of her friends. Her purpose, her training, and the reason for her suffering, all within her grasp.
She lifted her dagger, took two running steps, and plunged her dagger. . . .
. . . into the Mountain's neck and out through his throat.
He gave a surprised gurgle, then the sword slipped from his hands. The Hound let out a gasp as the Mountain fell on top of him, knocking the breath out of his lungs. He lay still, panting.
"Are you alive?" Arya asked calmly.
The Hound kicked his brother's body off him and sat up. "Bloody fucker," he grumbled. "Don't expect me to thank you."
"You're bleeding," Arya said.
The Hound looked himself over. "Just a cut in my leg," he growled. "I'm bruised, but I'll live." His eyes softened as he looked at Arya. "I'm sorry," he said grudgingly. "That was your chance to cross Cersei off your list."
"It's all right," she answered, with a hint of a smile. "I fight for the living."