Basically the ex guild master and the party members all died or just disappeared.
Ex guild master was betrayed by a woman. One day he found a prostitute resembling the woman. He wanted revenge on this unrelated prostitute. He promised to free her, but then stopped visiting her. He visited under a different disguise and was enjoying seeing her despair. He eventually bought the brothel and made her the manager. Eventually the brothel became a place for sadistic clients (killing, torture). He was simply enjoying the suffering the manager went through handling the prostitues' corpses.
One day he found Shiori and determined to make her his workers. The manager could no longer take it, especially when she imagined the torture a rare girl like Shiori would be put through. She reported the brothel, and the place closed down after a raid.
The ex guild master managed to run thanks to his disguise, but still determined to take Shiori. He pressured the Akatsuki party to exploit and trap her, then planned to sell her to slavers.
But after Shiori returned from near death, Zack and the police investigate the case and managed to link the ex guild master with the brothel. Zack killed him by his own hand.
Akatsuki party was originally a good party. But after being under the ex guild master pressures, they simply broke down. Not much details, but one died during quest, two killed each other, one is unknown but likely die, one excommunicated.