  We present a self-contained overview of GR-Athena++, a general-relativistic
magnetohydrodynamics (GRMHD) code, that incorporates treatment of dynamical
space-time, based on the recent work of (Daszuta+, 2021)[49] and (Cook+,
2023)[45].
  General aspects of the Athena++ framework we build upon, such as oct-tree
based, adaptive mesh refinement (AMR) and constrained transport, together with
our modifications, incorporating the Z4c formulation of numerical relativity,
judiciously coupled, enables GRMHD with dynamical space-times.
  Initial verification testing of GR-Athena++ is performed through benchmark
problems that involve isolated and binary neutron star space-times. This leads
to stable and convergent results. Gravitational collapse of a rapidly rotating
star through black hole formation is shown to be correctly handled. In the case
of non-rotating stars, magnetic field instabilities are demonstrated to be
correctly captured with total relative violation of the divergence-free
constraint remaining near machine precision.
  The use of AMR is show-cased through investigation of the Kelvin-Helmholtz
instability which is resolved at the collisional interface in a merger of
magnetised binary neutron stars.
  The underlying task-based computational model enables GR-Athena++ to achieve
strong scaling efficiencies above $80\%$ in excess of $10^5$ CPU cores and
excellent weak scaling up to $\sim 5 \times 10^5$ CPU cores in a realistic
production setup. GR-Athena++ thus provides a viable path towards robust
simulation of GRMHD flows in strong and dynamical gravity with exascale high
performance computational infrastructure.
