How did Leto II become a superhuman?
When Leto II journeyed into the desert to find the preacher, he got captured by a group of outlawed Fremen. Realizing that he was in danger, Jessica sent Gurney Halleck to infiltrate the group. With suggestions from Gurney, Leto got forced into a spice trance. After the spice trance, Leto II escaped his captors and fused himself with a school of sand trout, becoming superhuman.