Since Sheeana could sense the needs of the sandworms in the holding cargo bay of the Ithaca brought with them off of Chapterhouse, she felt the worms knew they were destined to wander the stars for decades. As such, they were not hostile to one another, and they as a group community, decided that Monarch would be their leader. Monarch was a ferocious sandworm, and was over sixty meters in length, the longest of any of the worms held in the captive cargo hold.
After the birth and raising of the ghola of Leto II, Leto visited Monarch, and began a strange communion with him, and the rest of the giant beasts. Leto could sense the "pearl of awareness" of himself that was inside of the worms, and could at times sense their unhappiness, joy, or pain.
As Leto approached the age of adulthood, he went into the sandhold and began communing with Monarch again. The ghola of Thufir Hawat, who was in actuality an Enhanced Face Dancer, stepped in to guard Leto, as Leto began to speak to Monarch. As the Face Dancer had assumed Hawat's identity for so many years, the Dancer actually had lost his sentience that he was actually a Face Dancer. As soon as the Dancer approached Monarch and the other sandworms, the worms killed Hawat-Face Dancer, and the Dancer reverted to its' original clone-like vestige. This alerted the crew that there were indeed at least one other Face Dancer aboard the vessel.