Iarno was a member of the Lords' Alliance faction, he received a mission from the faction master to investigate people diappearance around Phandalin.
Once he got there he found out The Black Spider was responsible for kidnapping people. The Black Spider approached Iarno and offered him a job, keep Phandalin's denizens in check.
He offered him a precious Glass staff and the control over Phandalin. Iarno agreed and started a secret group inside the basement of Tresendar Manor in eastern side of town called the Redbrands.
He never used names, but since his staff was so unique, he was known to everyone only as Glasstaff.