Dumisani Blasius Khumalo "was killed by unknown gunmen last night, shortly after attending the Reed Dance in Nongoma," Sihle Zikalala, a member of the regional legislature, said in a statement.
Khumalo, the king's chief advisor, was responsible for settling disputes within the Zulu community.
"He was very loyal to the royal family and he worked closely with MisuZulu's father before," said Khaya Ndwandwe, a Zulu culture expert.
Some 10,000 people gathered Saturday in the small town of Nongoma, a Zulu stronghold in the southeastern KwaZulu-Natal province, for the traditional "reed dance".
During the centuries-old ritual, the girls present themselves to the king of the country's largest ethnic group, which numbers 11 million Zulus, almost one in five South Africans.
It was the first reed ceremony for King MisuZulu Zulu.
Opponents of the new ruler had issued threats and predicted a "bloodbath" if the celebration went ahead.
Since the death of Goodwill Zwelithini last year after a reign of 50 years, a dispute over succession has raged.
In South Africa, traditional rulers and chiefs are recognised by the constitution and exercise significant moral authority.
MisuZulu kaZwelithini, recognized by President Cyril Ramaphosa, was born to the favorite and third wife of Goodwill Zwelithini.