In this chapter, for the first time ever, Jack is the one who uses the conch to call for an assembly.
The reason that he wants to call an assembly is because he wants to get rid of Ralph as leader. He wants the boys to throw Ralph out and put himself in as leader instead.
At this point, however, the boys are not ready to do this. They vote to keep Ralph. At that point, Jack decides to leave the group along with his hunters. He says that he will not play along with them any longer.
In Chapter Eight of Lord of the Flies, Jack steals the conch from Piggy and blows it to call an assembly. During this meeting, he inspires fear in the other boys by telling them that there is a beast roaming around and terrorizing the island. He then attempts to have Ralph removed from his role as the leader due to his cowardice.
Despite this betrayal, the boys do not vote Ralph out of his position; instead, Jack decides to start his own tribe and storms off toward the beach, extending an open invitation for others to follow him. Those who do become ferocious hunters who kill a sow and make a ritual sacrifice of its head on a stake to the alleged beast of the mountain.