Siaynoq was a religious orgiastic, ecstatic sexual experience involving Fish Speaker leaders who were called to Arrakis every ten years to commune with the God Emperor Leto II in his Fesitval City of Onn.
Every 10 years the women leaders of were gathered together from all over the Imperium, in a large amphitheater that Leto had built on Arrakis. The women were arranged in a giant circle, and the God Emperor would appear. After 3 to 4 hours of chanting Leto's name, and hearing him speak to them in intimate ways, the women would crescendo in an ecstatic ritual with Leto as their focus.
It is rumored that after Leto's death, and the subsequent colonization of planets by the Fish Speakers during The Scattering, that the remnant and corruption of the concepts of Sianyoq, became one of the foundations of the Honored Matres sexual enslavement of males.