In order to understand the conditions that lead to a highly magnetized, relativistic plasma becoming unstable, and in such cases how the plasma evolves, we study a prototypical class of magnetostatic equilibria in which the magnetic field satisfies ∇×B=αB, where α 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 time scales 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.