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Gandalf’s role was not one of overt dominance but rather to guide and inspire collaboration among the various races. The Valar’s directive required him to walk amidst mortals in a "humble form," refraining from openly demonstrating immense magical power.
Additionally, Gandalf's body, despite being infused with divine spirit, was still subject to the frailties of flesh. As seen in his confrontation with the Balrog, he ultimately succumbed to the limits of his body and was faced with death.