While casting Demilich, if you are required to pay an additional cost that includes a generic mana component, Demilich's cost reduction effect can apply to it. For example, if you're required to pay an additional {1} to cast Demilich (perhaps because of Sphere of Resistance), and you've cast five or more instant and/or sorcery spells this turn, you would pay {0} to cast Demilich.