  We present AsterX, a novel open-source, GPU-accelerated, fully general
relativistic magnetohydrodynamic (GRMHD) code designed for dynamic spacetimes
in 3D Cartesian coordinates, and tailored for exascale computing. We utilize
block-structured adaptive mesh refinement (AMR) through CarpetX, the new driver
for the Einstein Toolkit, which is built on AMReX, a software framework for
massively parallel applications. AsterX employs the Valencia formulation for
GRMHD, coupled with the 'Z4c' formalism for spacetime evolution, while
incorporating high resolution shock capturing schemes to accurately handle the
hydrodynamics. AsterX has undergone rigorous testing in both static and dynamic
spacetime, demonstrating remarkable accuracy and agreement with other codes in
literature. Benchmarking the code through scaling tests on OLCF's Frontier
supercomputer, we demonstrate a weak scaling efficiency of about 67%-77% on
4096 nodes compared to an 8-node performance.
