Complex sentence: He begins to regard Harry as his new master and fulfills his chores dutifully; and, in The Second Battle of Hogwarts, Kreacher rallies the Hogwarts house-elves in the names of Harry and Regulus, and leads them into battle against the Death Eaters.
Split into simpler sentences: He begins to regard Harry as his new master and fulfills his chores dutifully. Later, in The Second Battle of Hogwarts, Kreacher rallies the Hogwarts house-elves in the names of Harry and Regulus, and leads them into battle against the Death Eaters.