  We report on a new capability added to our general relativistic
radiation-magnetohydrodynamics code, Cosmos++: an implicit Monte Carlo (IMC)
treatment for radiation transport. The method is based on a Fleck-type implicit
discretization of the radiation-hydrodynamics equations, but generalized for
both Newtonian and relativistic regimes. A multiple reference frame approach is
used to geodesically transport photon packets (and solve the hydrodynamics
equations) in the coordinate frame, while radiation-matter interactions are
handled either in the fluid or electron frames then communicated via Lorentz
boosts and orthonormal tetrad bases attached to the fluid. We describe a method
for constructing estimators of radiation moments using path-weighting that
generalizes to arbitrary coordinate systems in flat or curved spacetime.
Absorption, emission, scattering, and relativistic Comptonization are among the
matter interactions considered in this report. We discuss our formulations and
numerical methods, and validate our models against a suite of radiation and
coupled radiation-hydrodynamics test problems in both flat and curved
spacetimes.
