The Hromi Cluster was a region of space near the Acamar system, where a number of M-class planets were located, including Gamma Hromi II. The Federation had charted, but not explored, several of these planets. In 2366, the USS Enterprise-D entered the Hromi Cluster to locate a Gatherer outpost. (TNG: "The Vengeance Factor")
In the backstory to Star Trek Online, in 2404 the Klingon Empire's Chancellor J'mpok (β) invoked ancient Klingon claims to the Hromi Cluster and demanded that all Federation citizens evacuate within three months. The Federation refused, and in 2405 the region became the first battleground of the Third Federation-Klingon War (β).
In the game proper, the cluster is located in the Pi Canis sector block (β) and features various missions and duty officer assignments.