  A next generation of Compton and pair telescopes that improve MeV-band
detection sensitivity by more than a decade beyond current instrumental
capabilities will open up new insights into a variety of astrophysical source
classes. Among these are magnetars, the most highly magnetic of the neutron
star zoo, which will serve as a prime science target for a new mission
surveying the MeV window. This paper outlines the core questions pertaining to
magnetars that can be addressed by such a technology. These range from global
magnetar geometry and population trends, to incisive probes of hard X-ray
emission locales, to providing cosmic laboratories for spectral and
polarimetric testing of exotic predictions of QED, principally the prediction
of the splitting of photons and magnetic pair creation. Such fundamental
physics cannot yet be discerned in terrestrial experiments. State of the art
modeling of the persistent hard X-ray tail emission in magnetars is presented
to outline the case for powerful diagnostics using Compton polarimeters. The
case highlights an inter-disciplinary opportunity to seed discovery at the
interface between astronomy and physics.
