  We report on the thermodynamic and transport properties of the rare-earth
Zintl compound Eu$_5$Sn$_2$As$_6$, which orders as a canted antiferromagnetic
magnetic semiconductor at 10.3~K. The system also displays a complex cascade of
magnetic phases arising from geometric and magnetic exchange frustration, with
high sensitivity to the application and direction of small magnetic fields. At
low temperature, Eu$_5$Sn$_2$As$_6$ exhibits negative colossal
magnetoresistance of up to a factor of $6\times10^3$. This may be a lower bound
as the conductivity appears to be shunted by an unknown conduction channel,
causing the resistivity to saturate. Mechanisms for the low temperature
saturation of resistivity are discussed.
