The European Spallation Source (ESS) will be the world's brightest neutron source and will open a new intensity frontier in particle physics. HIBEAM/NNBAR is a proposed two-stage program to exploit the unique potential of the ESS to address open questions in modern physics including the nature of dark matter and the origin of the matter-antimatter asymmetry. The first stage, HIBEAM, includes the first search in thirty years for free neutrons converting to antineutrons and searches for sterile neutrons, ultralight axion dark matter and nonzero neutron electric charge. Sensitivity increases of an order of magnitude compared to current work are expected. Beyond HIBEAM, NNBAR is a dedicated search for free neutrons converting to antineutrons with a sensitivity increase of three orders of magnitude. This talk outlines the capabilities, design, infrastructure, and scientific potential of the HIBEAM/NNBAR program.