Antagarich is the name of the continent on which most of the action in Heroes 3 and its expansions take place. The main country is Erathia, but there are also AvLee, Bracada, Deyja, Eeofol, Krewlod, and Tatalia. There is also the island of Vori on the North-West and Nighon on the South-East.
Most of Antagarich was once ruled by the Wizard-Kings of Bracaduun. A legendary Barbarian, Jarg, united all the tribes of the Wastelands (Krewlod) and conquered the entire southern-half of Antagarich. However, after he died of old age, his heirs squabbled for the throne and dismantled the nation in their squabbles, allowing the Wizard-Kings to rise again and reestablish their empire. They oppressed Barbarian culture especially, attempting to suppress the history and legend of Jarg in fear that heroes would try emulating Jarg; and had nearly succeeded in that task when Tarnum rebelled and repeated history.
Tarnum, however, was not as wise as Jarg. His reign was especially ruthless and brutal, forever twisting Barbarian culture, and Tarnum himself turned out to be as much a tyrant as the Wizard-Kings were. Rion Gryphonheart would soon lead an uprising against the Barbarian King, and vanquish him in single combat. With Tarnum's death, his empire crumbled quickly, allowing Rion Gryphonheart to create the new nation of Erathia which quickly dominated most of the continent.
Two other continents exist on the same planet, named Enroth and Jadame. Enroth is the homeland of King Roland Ironfist and the setting where the first two Heroes of Might and Magic take place. Jadame is only seen in Might and Magic VIII: Day of the Destroyer and there is no mention of it in the Heroes series, though Regna, which is represented by the Cove town in Horn of the Abyss, is considered to be from there.