Caravan 208B
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Confusion; Troubleshooting; Workload
Aircraft Equipment Problem Critical; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Provided Assistance; Flight Crew Regained Aircraft Control; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Departure Airport; Flight Crew Overcame Equipment Problem; General Flight Cancelled / Delayed
Aircraft
Aircraft X departed ZZZ with its planned destination of ZZZ1. The aircraft taxied for runway XXR and departed on an IFR clearance. By all indications the takeoff was normal and nothing unusual occurred. At approximately 200 feet the Garmin autopilot Self - Engaged and put the airplane into a steep descent. I immediately pushed the Auto-Pilot (AP) disconnect switch; which (briefly) disconnected the AP before it re-engaged. This occurred two or three times consecutively as I tried to troubleshoot the issue. I then pushed and held the AP disconnect switch for 5-6 seconds with no success. (All the while the airplane continued to make attempts at descents.) Additionally; I tried the CWS (Control Wheel Steering) button to attempt to regain control over the autopilot. The current weather in ZZZ was IMC; with ceilings of 600 feet and 6 miles visibility with mist. I was on an IFR flight plan; and made the decision to request priority handling with ZZZ tower. I remained below the weather to return to the field and land runway XXR. To regain control over the autopilot; I turned off the avionics; and landed without incident. The cause of the event is unknown at this time while maintenance is troubleshooting the issue. At this time it is unknown (from a pilot's  perspective) as to what could have prevented the malfunction of the autopilot; and cause it to self-engage.
C208 pilot reported autopilot malfunctions in IMC conditions and elected to perform an air turn back and a precautionary landing.