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The Tregharl Cluster was a star cluster located somewhere in the galaxy's Alpha or Beta Quadrant, a region of space that contained Klingon colonies in the 23rd century.
In the year 2290, spacecraft traveling from the Klingon Homeworld to these colonies often crossed the Federation-Klingon Neutral Zone, violating the Organian Peace Treaty. (TOS - The Captain's Table novel: War Dragons)