JOZINI, South Africa—In her battle to secure the rights to her dusty homestead, Hluphekile Mabuyakhulu has joined a small group of South Africans squaring off with a powerful landlord: the king of the Zulu nation.

A 1994 deal sealed in the final days of the apartheid government gave King Goodwill Zwelithini, the ruler of South Africa’s largest ethnic group, control of more than 11,000 square miles of land, some 2.9 million hectares, a property nearly the size of Maryland. Through a private trust, King Zwelithini is one of...