AU where there is a mole in the team… but he wasn't at the time of Sportsmaster's warning. Now he is exposed when the team (Nightwing, Robin, Miss Martian, Superboy, Blue Beetle, Lagoon Boy, Kid Flash and Impulse) is captured by the League of Assassins. Kid Flash came back a bit after Artemis disappeared, and Miss Martian didn't find out about Aqualad.
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Nightwing would never admit it, but he didn't see how the team would get out of this alive. They were in a dark, windowless cavern, surrounded by assassins, and Aqualad, Tigress, Sportsmaster and Ra's AlGhul were here. He automatically stepped in front of the Team, while Superboy and Miss Martian stepped next to him, just a bit behind him.
"Ra's." Nightwing nearly growled.
"Nightwing. I have to say, I am disappointed in both you and the detective. However, this does show that my choice in heirs was the best I could have made." Ra's responded, sneering at Nightwing.
"This place should only have had a few ninjas in it. This was only supposed to be a small base." Nightwing defended, feeling his heart beat faster at the mention of an heir. Who was the heir?
"Until a few weeks ago, it was. Once we received word that you were bringing your little team, I immediately had ninja moved here."
At Nightwing's confused, worried look, Sportsmaster said "I warned you there was a mole. I just didn't say they weren't on the roster. They were a mole before any of you even knew they existed, and he has been very useful, isn't that right, Artemis." He grabbed Tigress' arm, dragging her in front of him, and yanked off the glamour charm. The team gasped as Tigress turned into Artemis.
"Hey, Dad." Artemis growled. Sportsmaster threw her to the team. Nightwing and Kid Flash each grabbed one of her arms to keep her from falling.
"Aqualad was never the mole, except for you, Nightwing." Sportsmaster shot Aqualad in the shoulder, causing him to let out a small yell. Two ninja threw him at the team where Artemis and Nightwing helped him stay upright. The team looked shocked and horrified. "You organized it, so you will suffer severely for it." The team got between Nightwing and where Ra's AlGhul and Sportsmaster were standing.
"Now, now, no need to get between us and him; after all, it's not one of us who has a sword to his neck." Ra's smirked. The team spun around and gasped when they saw Nightwing looking at the sharp katana that was at his neck… with Robin holding it.
"R-robin… Why?" Nightwing's voice cracked with emotion.
Robin glared, "I lived next to you and Batman for years, followed you guys at night, but you never noticed. I told you my parents weren't good people. Father and Mother started getting drunk and high when I was five. He beat me. You and B saw the hand-shaped bruises, you just ignored them. The beatings got worse and worse, but you did nothing. YOU COULD HAVE STOPPED HIM!" Robin yelled as he dug the sword in enough to draw blood. "You could have stopped him. You could have stopped me." Robin whispered. "I was the one that killed her, you know. Father raped me while Mother laughed. I decided I'd had enough and enjoyed snapping her neck, slitting her throat, putting her through what I went through."
"You said you saw the murder and murderer. You said you didn't recognize him." Nightwing said, looking horrified.
"Ha. The best part: I didn't even have to lie. There was a mirror where I killed her. I looked in the mirror and saw me kill her. The me I saw in the mirror, though, wasn't anyone I'd ever seen before. They were allowed to have an opinion, they stood up for themself, they weren't taking that shit anymore, they had power. I'd never seen that side of me. I liked it, and it's not like the police would ever suspect a young, well-behaved socialite child of murdering his mother. Father left on another trip not even a week after her murder: leaving me home alone. I ran out of food and went to suicide slums to cage fight, like I usually did when they left. The League of Shadows found me, took me in, trained me. They gave me the chance to take down not only the Justice League and Young Justice, but also the Bats who could have saved me!" Robin quickly pushed Nightwing into the rest of the team and, when they looked, he was gone.
They turned to Ra's and Robin was right next to him, katana in a sheath on his back. "He is the best I've ever seen; violent, but cunning enough to get what he needs before slowly killing his target. Be quick, Timothy, they will be executed at dawn." Ra's and Sportsmaster left.
Nightwing looked heartbroken and betrayed, worse than anyone else on the team. Robin saw him and smirked, saying in a patronizing tone, "Oh, don't feel too bad, Nightwing: I manipulate everyone." At that, he turned and left.
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Dun, dun, dunnnnn.