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The IKS Heart of Kahless was the Klingon flagship in the 2380s in an alternate timeline.
In 2388, as part of Project Looking Glass, Martok took the Heart of Kahless and a Klingon fleet into the mirror universe and fight towards Bajor, planning to cross back into the primary universe to engage the Grigari. However, the Grigari themselves transported to the mirror universe and destroyed the Heart of Kahless and the Klingon armada. (DS9 - Millennium novel: The War of the Prophets)