Book 5 forest is divided into territories and cats can choose to follow Tall Shadow. Shadowclan is formally given its name in Book 6, so technically she doesn’t officially become the “leader” until then
leaderChange After Shaded Moss…
After Shaded Moss is killed on the Thunderpath, Tall Shadow, later Shadowstar, becomes the new leader of the former Tribe cats. She eventually settles with her group in the pine forest and becomes the first leader of ShadowClan.