Xerrath was a world destroyed by the Burning Legion as a show of power during their enslavement of Xoroth.
Warlock players find a codex while undertaking their "green fire" quest. After speaking to their Observer minion, they are informed of the following:
Of course! This codex is written in the ancient Nathrezim tongue. Nathrezim, or Dreadlords, 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. The tome is useless, I'm afraid. No human warlock could open such a gateway alone.