Last year, ESA initiated the next strategic planning cycle to identify the driving science priorities in the 2035-2050 time frame. I will present the white paper that we submitted to this call, advocating for a mega-pixel, wide field of view X-ray microcalorimeter optimized for high resolution spectroscopy the soft X-ray band. Such a mission would achieve the ambitious goal of surveying the thermodynamics, chemical composition, and kinematics of all warm-hot baryons in the cluster outskirts, circumgalactic media of L* galaxies, and throughout the diffuse strands of the cosmic web, over a significant fraction of the entire sky, providing the first tomographic map of the dominant form of matter in the Universe.