  We describe a new algorithm to solve the time dependent, frequency integrated
radiation transport (RT) equation implicitly, which is coupled to an explicit
solver for equations of magnetohydrodynamics (MHD) using {\sf Athena++}. The
radiation filed is represented by specific intensities along discrete rays,
which are evolved using a conservative finite volume approach for both
cartesian and curvilinear coordinate systems. All the terms for spatial
transport of photons and interactions between gas and radiation are calculated
implicitly together. An efficient Jacobi-like iteration scheme is used to solve
the implicit equations. This removes any time step constrain due to the speed
of light in RT. We evolve the specific intensities in the lab frame to simplify
the transport step. The lab-frame specific intensities are transformed to the
co-moving frame via Lorentz transformation when the source term is calculated.
Therefore, the scheme does not need any expansion in terms of $v/c$. The
radiation energy and momentum source terms for the gas are calculated via
direct quadrature in the angular space. The time step for the whole scheme is
determined by the normal Courant -- Friedrichs -- Lewy condition in the MHD
module. We provide a variety of test problems for this algorithm including both
optically thick and thin regimes, and for both gas and radiation pressure
dominated flows to demonstrate its accuracy and efficiency.
