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Novel electronic phenomena frequently form in heavy fermion systems because of the f-electrons’ localized and itinerant nature. When magnetically ordered, the f moments are expected to be frozen and localized, decoupled from the Fermi surface. It remains ambiguous whether a Kondo-lattice can emerge from deep inside the magnetic phase or simply breaks down below T_N. Using spectroscopic imaging with the scanning tunneling microscope complemented by neutron scattering, we probe the electronic states in the antiferromagnetic USb₂ as a function of temperature. We visualize an antiferromagnetic gap at high temperatures (T<T_N~200K) in which Kondo hybridization gradually develops below T^coh~80K near the Fermi energy. At T*=45K we find an electronic transition through the abrupt emergence of non-trivial sharp 5f quasiparticles, whose appearance agrees with the sudden release of entropy seen in specific heat. Our findings demonstrate the emergence of Kondo physics from inside USb₂'s antiferromagnetic phase, spectroscopically establishing the dual nature of f-electrons, and reveal an enigmatic electronic transition, which may share connection with the ‘hidden order’ phase of URu₂Si₂.