Series Two

Series Two is a term, according to Acheras the Custodian, used by titan scientists to refer to the successful second attempt at crafting a mortal race of guardians from living stone for Azeroth. The troggs (presumably Series One) were the mistaken first attempt. When the titans saw how brutal and misshapen the troggs were, they buried them in a vault under Bael Modan.

The Lore Keeper of Norgannon in Uldaman mentions a second variant in its exposition, which may be the same thing, or related. The second variant matrix is the earthen who destabilized into dwarves.