The complex, multiscale behavior of hot magnetized plasmas comprising most of the visible matter in the universe is governed by collective nonlinear plasma processes (e.g. magnetic reconnection, turbulence, shocks) involving energy exchange between the plasma and electromagnetic fields. This dynamics often drives nonthermal particle acceleration, powering intense flaring emission across the electromagnetic spectrum in numerous astrophysical sources, including neutron stars and black holes. However, the physics of the plasmas around these relativistic objects is richer and more complex than traditional (i.e. heliospheric or fusion) plasma physics, and involves relativistic effects, strong interaction of plasma with radiation, and QED effects like electron-positron pair creation.

The plasma physics of relativistic objects in the presence of these effects is an exciting frontier of modern physics. Recent observational breakthroughs, like the discovery of Fast Radio Bursts and the Event Horizon Telescope's images of black holes, challenge our understanding and motivate this exploration. Rapid progress in this burgeoning field is enabled by the advent of novel relativistic, radiative and QED kinetic plasma simulations. In addition, new powerful laser facilities are opening exciting avenues for experimental study of extreme plasma regimes. This program will bring together experts in observational and theoretical high-energy astrophysics, plasma theory, and high-energy-density laboratory physics to foster dialog and collaboration between these traditionally disparate communities. It will catalyze transformative progress in understanding extreme astrophysical plasmas and will explore how laboratory experiments can probe these exotic regimes.