It was destroyed by the Legion

The quest series begins by looting Sealed Tome of the Lost Legion, currently from a Green Test Infernal Kingdoms Of Camelot Gems  by the Shado-Pan Garrison in Townlong Steppes. (Ghostcrawler has stated that it will be rare to get on live, but not super-rare.) Fuse together Sealed Tome of the Lost Legion with Healthstone to get An Unusual Tome.
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Speak to your demonic minions to Cheap Kingdoms Of Camelot Gems  learn more about the codex. Your Voidwalker and Felhound are the chattiest:◦
Voidwalker: I cannot read the words…but the magic points to a world that exists no longer. It was destroyed by the Legion.
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Observer: Of course! This codex is written in the ancient nathrezim tongue. Nthrezim, or the Dreadlords,  Cheap Fast & Furious 6: The Game Gold  as you call them, were once an enlightened and powerful race. Their skill in summoning and gateway magics was unsurpassed by any in the Great Dark Beyond. This particular tome contains instructions to open a gateway to another world. Sadly, I fear that world no longer exists. It was destroyed as a show of power during the enslavement of Xoroth. Buy Fast & Furious 6: The Game Gold   That tone is useless, I'm afraid. No human warlock could open such a gateway alone.