The pilot said that he failed to maintain directional control after landing in dark night visual conditions. The airplane exited the right side of the runway onto a grassy area, struck the edge of a paved taxiway, and then came to a full stop on the taxiway with the engine still operating. The pilot added power to begin taxiing and discovered he could steer the airplane only to the left. The pilot reported that he then added power to begin a taxiing left turn onto the grass “in order clear the runway.” While taxiing on the grass surface the nose wheel inadvertently went down a slope and the propeller struck an obstruction. The impact caused substantial damage to the forward fuselage structure and the firewall.