It is a formal convocation of Fremen leaders to witness a combat that determines tribal leadership.
There are no more uses of "convoke" in the book.
Show samples from other sources
Colenso was the object of acrimonious debate wherever the clergy gathered, and Stanley once made a ringing Convocation speech in his defense, asking that body was it aware that Colenso was the only colonial bishop who had bothered to translate the Bible into Zulu, which was rather more than the rest had done.
Harper Lee -- Go Set a Watchman
He did not need to tell the Convocation he had failed.