The first thing known about this piece is a rumor shared by U’Selekma. See the log A Meeting With U’Selekma for the rumor.
Then the 3 scholar priests hired by Azariel uncover more information, secreted in a scroll sent to Sekti-Abtu by Rystrom Khejas. This is recapped in the log Giants and the Arm.
It is rumored that the Left Arm is in the kingdom of the Giants, the Nimro Kingdom, and is held by the King himself, Sunder Blackrock.