B757 First Officer reported the aircraft initiated an uncommanded aggressive pitch downwards while descending on the arrival but the flight crew were able to regain control and landed safely.

Narrative

While descending on the ZZZZZ Arrival into ZZZ; around 8000 feet; we received an EICAS warning: UNSCHD STAB TRIM. Simultaneously the airplane initiated an aggressive 4-degree nose down. The autopilot disconnected and the autopilot disengage alarm sounded. I silenced the alarm and read the associated message while the Captain immediately regained control and started hand flying the aircraft. We estimated a 400 to 600 foot loss; but did not exceed any published altitude or airspeed restrictions on the arrival. I reviewed the associated non-normal checklist and we discussed the next steps. Once the autopilot was off; the EICAS message extinguished; and the aircraft was under control. The Captain mentioned the only degradation was slightly heavier control inputs were needed. With that determined; we both agreed running the checklist and identifying which specific system was inoperative would unnecessarily add to our workload. We concurred that best course of action was to continue the approach; hand flown; and land. A normal approach and landing was accomplished; followed by a normal taxi to the gate. Upon arrival; Maintenance Control was notified; an Electronic Log Book (ELB) entry was made; and the Captain filed a report as well as contacted the Chief Pilot.