That is a nice sestertius and certainly scarce sestertius. Great coin!
It is assumed that this sestertius was issued to commemorate Victories achieved by Domitianus in repelling and pushing invasions in 83-85 a.d of the Chattii, a germanic tribe which inhabited the central Rhein area. Most of the fighting took place in the areas around Mogontiacum and Aquae Mattiacorum (present day Mainz and Wiesbaden).
This sestertius in my collection was also part of that issue. It shows Kaiser Domitian on horse spearing a fallen germanic warrior. On this coin, as on yours the warriors carry a 6-sided shield typical of those used by the germanic tribes of that era.

