  Here we investigate antiferromagnetic Eu$_{5}$In$_{2}$Sb$_{6}$, a
nonsymmorphic Zintl phase. Our electrical transport data show that
Eu$_{5}$In$_{2}$Sb$_{6}$ is remarkably insulating and exhibits an exceptionally
large negative magnetoresistance, which is consistent with the presence of
magnetic polarons. From {\it ab initio} calculations, the paramagnetic state of
Eu$_{5}$In$_{2}$Sb$_{6}$ is a topologically nontrivial semimetal within the
generalized gradient approximation (GGA), whereas an insulating state with
trivial topological indices is obtained using a modified Becke-Johnson
potential. Notably, GGA+U calculations suggest that the antiferromagnetic phase
of Eu$_{5}$In$_{2}$Sb$_{6}$ may host an axion insulating state. Our results
provide important feedback for theories of topological classification and
highlight the potential of realizing clean magnetic narrow-gap semiconductors
in Zintl materials.
