Who is Hedley Tuek from Heretics of Dune and what is their importance?

The tall, stout, imposing gray-haired High Priest of the Divided God and titular ruler of the planet Rakis, Tuek, receives a report about a young girl, Sheeana, who has ridden in from the desert atop a great sandworm. Tuek allows a test in the desert, which the terrified girl survives. Tuek vacates his quarters and moves her in, believing that she is God's tool for weeding evil out of the priesthood. Sheeana gradually realizes she has great power over those around her, particularly Tuek, who she addresses by his first name. Knowing that more than one predecessor has died mysteriously in the desert, Tuek nevertheless insists that the High Priest must be the final interpreter.