According to Xal'atath, the Battle for K'tanth was a battle that occurred long ago in the land now known as the Broken Shore. It is not stated what K'tanth is or was.
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Xal'atath makes several other remarks regarding the Broken Shore and its connection to the Old Gods. The dagger claims that the Tomb of Sargeras pales in comparison to the grandeur of what stood there long ago. It explains that Thal'dranath was always a place of power, and that Aegwynn was drawn there, as were the elves, the trolls, and something else before them, likely the Old Gods. Furthermore, Xal'atath claims that the land had not seen "such carnage" since the Battle for K'tanth, which occurred "long ago", and that it was "here" in ages past that N'Zoth lost a great battle to Y'Shaarj, though the defeat ultimately worked in N'Zoth's favor. Thus, it can be speculated that the battle between N'Zoth and Y'Shaarj was fought over this place of power and the great edifice that stood there long ago might've been named K'tanth, or alternatively, "K'tanth" what was situated beneath it.
The edifice might have been destroyed during this battle, by the elemental armies that brought down the temples of the Old Gods, or by the titan-forged who destroyed the Black Empire.
^Xal'atath whispers: I don't believe these lands have seen such carnage since the Battle for K'tanth. Such a long time ago...