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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 exposition found in the Horde Player's Guide, while similar to that given by the Lore Keeper of Norgannon in World of Warcraft, does not mention the earthen turning into troggs. It refers to the troggs and earthen as two as separate experiments.
- While this seemingly contradicts both the Lore Keeper of Norgannon and the Tribunal of Ages, the existence of stone troggs that were exposed to the Curse of Flesh separately from the earthen actually fits more with this explanation.