Iron-bearing oxides undergo a series of pressure-induced electronic, spin, and structural transitions that can cause seismic anomalies and dynamic instabilities in Earth's mantle and outer core. We employ x-ray diffraction and x-ray emission spectroscopy along with density-functional theory+dynamical mean-field theory calculations to characterize the electronic structure and spin states, and crystal-structural properties of wüstite (Fe1-xO)-a basic oxide component of Earth's interior-at high pressure-temperature conditions up to 140 GPa and 2100 K. We find that FeO exhibits complex polymorphism under pressure, with abnormal compression behavior associated with electron-spin and crystallographic phase transitions, and resulting in a substantial change of bulk modulus. Our results reveal the existence of a high-pressure phase characterized by a metallic high-spin state of iron at the pressure-temperature conditions near to those of Earth's core-mantle boundary. The presence of high-spin metallic iron near the mantle can significantly influence the geophysical and geochemical properties of Earth's deep interior.