In this talk, I will discuss our recent result. Either there is a counterexample to black hole uniqueness, or the following statement holds. Axisymmetric, complete, simply connected, maximal initial data sets for the Einstein equations of nonnegative energy density with ends that are either asymptotically flat or asymptotically cylindrical, admit an ADM mass lower bound given by the square root of total angular momentum. Moreover, equality is achieved only for a constant time slice of an extreme Kerr spacetime. The proof is based on a novel flow of singular harmonic maps with hyperbolic plane target, under which the renormalized harmonic energy is monotonically nonincreasing. Relevant properties of the flow are achieved through a refined asymptotic analysis of solutions to the linearized harmonic map equations.