Multiple realities(covers information from several alternate timelines)
H'atoria was a planet located in the Beta Quadrant.
In 2016, a Vulcan ship crossed into Klingon space near H'atoria and was immediately destroyed by the Klingons. In the aftermath, Vulcans established a policy of firing first when encountering Klingons, earning their respect and eventually leading to the establishment of formal relations. (DIS: "The Vulcan Hello", "Magic to Make the Sanest Man Go Mad")
While it was not explicitly stated, this was strongly implied to have been the first contact between Vulcans and Klingons.
In 2256, H'atoria's location was labeled on the star chart "Alpha/Beta Quadrant Overview" in the ready room aboard the USS Discovery. It was located in the territory of the Klingon Empire. (DIS: "Magic to Make the Sanest Man Go Mad", "The War Without, The War Within")
A year later, its location was labeled on the star chart showing the trajectory of Lieutenant Spock's shuttle. (DIS: "An Obol for Charon")
As of the late 24th century, it was home to a small Klingon colony, which was located near the border with the Federation.
In a possible future timeframe, Worf was governor of H'atoria. (TNG: "All Good Things...")
This planet was only mentioned in dialogue. "ha-TORE-ee-uh" was the pronunciation for this planet's name from the script pronunciation guide. 
According to the novel A Singular Destiny, H'atoria was home to the Selsseress, a barely sentient amphibious race that had been conquered by the Klingons.