Cessna 441 pilot reported a loss of power in one engine while in cruise. The engine eventually failed inflight. The pilot followed the QRH procedures and landed safely.

Narrative

Began the day with pre-flight and running checks on both left and right engines. All temps and indications read normal. Ran the aircraft for 15 min on the ground for camera start up procedures and continued to have normal readings. Climbed out to 7;700 feet for the lower survey lines of a Boresite (camera calibration). Once at altitude power was pulled back to 900 feet LBS per side with the condition levers moved to cruise. Once under speed flaps were set to Take-Off to get the aircraft under 180 ground speed; required to get the images. Returned power to 1;500 foot LBS per side and condition levers back to climb; removed flaps; and climbed to 15;500 feet to complete the last two lines for the boresite. After all four lines were complete I turned NE bound to acquire a project over the states. Approaching the city I noticed a light torque drop on the left engine. It went from 1;500 to 1;400; so I checked lever friction settings and matched the torques with no changes in temps or fuel flow. The aircraft showed no signs of power loss at this point; and so we continued on course. Shortly after the torque dropped again but harder this time; down to 1;200 foot LBS. No changes occurred with increased throttle input. Disabled the flight computer to see if the plane would operate in manual mode; no change. Engaged the aux fuel pump; no change. RPM started decreasing and torque dropped slowly to 0 with noticeable surging. Reset the flight computer and checked the pumps again for assurance; applied auto ignitors to ensure the ignition system was operating correctly. With no probable cause or confirmation on what was causing the loss of power; I completed the shut down procedure to avoid catastrophic failure or fire. Told ATC of the situation; and was cleared direct ZZZ. Tower cleared all runways and approved landing on XX or XY. I was maintaining as much altitude as possible inbound to the airport and was unable to make XX due to altitude and speeds above gear down speed on descent. Proceeded upwind on XX making light right turns until inbound on approach for runway XY. Kept the speed above blue line and didn't reduce power until short final to avoid a VMC situation. Landed just past the 1000 ft. and pulled power to idle and condition levers to start taxi. Applied brakes and pulled off. Coordinated with ZZZ Ground for the company tug to retrieve the aircraft from the taxiway. Aircraft and personnel returned without any damages or injury. Maintenance is in the process of identifying the issue.