The ability to construct complex molecular architectures with precise control is critical for realizing molecule-based materials and functions. Using the assembly of a 60-metal complex of Er^III with histidine as an example, we demonstrate the rational assembly of otherwise synthetically elusive polynuclear lanthanide hydroxide clusters directed by the combined set of I⁻ and CO₃²⁻ as templates. We succeeded in the stepwise transformation starting from Er₁₂ to Er₆₀ by way of two key intermediates Er₃₄ and Er₄₈. The Er₁₂, Er₃₄, and Er₄₈ core motifs represent respectively 1/6, 1/2, and 3/4 of the complete sodalite cage of Er₆₀. This work, representing a rare example of rationally constructing high-nuclearity lanthanide clusters guided by judiciously chosen templates, is expected to stimulate more future efforts for the controllable synthesis of complex molecular or supramolecular architectures with unprecedented structural sophistication and possibly useful properties.