Omega's Talons
The Talons were originally a gang headquartered on the Omega space station throughout the entire Mass Effect series. They started out as a small-time gang composed primarily of turian mercenaries, offering protection services to the citizens of Omega. Later on, they would run a smuggling ring for red sand, a drug that affected only humans by gifting them a brief experience with telekinetic biotic abilities. The Talons members are easily identified by their red armor and red face paint.
In 2185, a plague was unleashed on Omega, affecting all species except for humans and the vorcha (a primitive, aggressive race). Turians, batarians and krogans from all mercenary gangs suffered heavy losses, resulting in a gap of power. The current Talon leader at the time, Derius, saw an opportunity for the Talons to hold more power aboard Omega and grabbed it.
By 2186, Nyreen has assumed leadership position of the Talons, transforming it into a highly organized military group dedicated to protecting the citizens of Omega rather than extorting them. When Aria is exiled from Omega by Cerberus' occupation of the asteroid, the Talons become the main resistance force under Nyreen's leadership. She rallies her forces to free citizens in Omega's Fumi district and the group eventually allies itself with Aria in order to take out Cerberus' leader.
After Nyreen dies by selflessly sacrificing herself to save innocent bystanders, an unnamed individual takes up leadership position in hopes of honoring Nyreen's memory. Once Cerberus is defeated and power is restored to Aria, the Talons become Omega's main security force.
Nyreen was happy in the turian military because she was able to help her people; she quit because she was no longer able to use her expertise to benefit others. After leaving Aria, it probably seemed like a natural choice to stay behind and take leadership over the turian-dominant mercenary group. Since many of the Talons seem to greatly respect Nyreen, it's possible that many of them had found themselves in the same situation that she did when arriving on Omega… Nyreen gave them (and herself) a new purpose in life.
Organizing the Talons into a lawful group must have made Nyreen happy; she was able to aid her people, give her fellow turians new purpose and help those in need.