  In order to understand the conditions which lead a highly magnetized,
relativistic plasma to become unstable, and in such cases how the plasma
evolves, we study a prototypical class of magnetostatic equilibria where the
magnetic field satisfies $\nabla \times\mathbf B = \alpha \mathbf B$, where
$\alpha$ is spatially uniform, on a periodic domain. Using numerical solutions
we show that generic examples of such equilibria are unstable to ideal modes
(including incompressible ones) which are marked by exponential growth in the
linear phase. We characterize the unstable mode, showing how it can be
understood in terms of merging magnetic and current structures, and explicitly
demonstrate its instability using the energy principle. Following the nonlinear
evolution of these solutions, we find that they rapidly develop regions with
relativistic velocities and electric fields of comparable magnitude to the
magnetic field, liberating magnetic energy on dynamical timescales and
eventually settling into a configuration with the largest allowable wavelength.
These properties make such solutions a promising setting for exploring the
mechanisms behind extreme cosmic sources of gamma rays.
