J-25 VI was the formerly inhabited sixth planet of the J-25 system. This was a class M planet. Prior to the mid-2360s, this planet was home to a highly industrialized civilization with cities linked by a system of roads. When the Borg attacked, the attacking ships removed cities, which caused rips to appear on the planet’s surface. In 2365, the Federation starship Enterprise scanned this planet. (TNG: "Q Who")
According to the Star Trek: Star Charts (pgs. 13, 81), the J-25 system was located in unexplored space in the Beta Quadrant. This system, also known as FGC-J25, was a single star system. Primary was a Class A star with a magnitude of +5, which was the same brightness as Sol.
According to the script of "Q Who", this is where the firm Borg connection would be made between this episode and the earlier TNG: "The Neutral Zone", "DATA: There is a system of roads on the planet which indicate a highly industrialized civilization. But where there should be cities there are only great rips in the surface. WORF: It is as though some great force just scooped all machine elements off the face of the planet. DATA: It is identical to what happened to the outposts along the Neutral Zone". (Scen 28)