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.
Update Thoth 28, 113. CrIsis recovers the Arm, and it is now held by Indaris. See the logs from the game date 11/21/15 for more.
It does 10D6 damage if user is confident of use.