Eclipse 500 pilot reported while in a descent they encountered severe turbulence resulting in the loss of control of the aircraft descending below the assigned altitude and the inability to reengage the Auto Pilot System. The pilot regained control of the aircraft and landed safely at the destination airport.

Narrative

At Approximately XA:08 Local time I experienced the most severe turbulence I have experienced in XX years of flying (the kind that smacks your head against the ceiling). I was descending per ATC instructions. I don't recall the assigned altitude but believe it was 14;000. Just above 14;000 I experienced the extreme turbulence which disconnected the auto pilot. I immediately pulled power to idle to reduce speed and the effects of the the turbulence. It literally felt like it was going to rip the wings off of the plane. I spent the next minute or so trying to get the auto pilot to reengaged. but every time I would engage it; the turbulence would disengage it. After a minute or so of attempting to engage the autopilot I abandoned that effort and just hand flew the airplane. During that minute or so I had descended to approximately 12;700 I was so shaken by the turbulence and the the fact that the auto pilot would not engage that I could not recall the assigned altitude; weather it was 14;000 or 12;000. I don't have access to the recording but I believe ATC instructed me to climb back to 14;000 because looking at my track log on foreflight I went back to 14;000 immediately. The rest of the arrival was uneventful. I landed at ZZZ in a strong cross wind and the controller told me to call a number for possible pilot deviation.