TRACON Controller reported aircraft descended below assigned altitude on approach resulted in a low altitude alert and a CFTT event.

Narrative

A BE9L was requesting ILS Approach into RWY XX at ZZZ Airport. At the time ZZZ Tower was closed. The aircraft was coming from the southwest direct ZZZZZ. I told the aircraft to expect radar vectors to final. I descended the aircraft to 060. I then issued the approach clearance; I emphasized the altitude assignment; 'Descend and maintain five thousand two hundred (052).' The aircraft read back the altitude assignment correctly. I continued to observe the aircraft as I saw a Low Altitude alert on the aircraft above the MVA. As soon as the aircraft passed 052 I issued a climb back to 052 and issued a low altitude alert. The aircraft immediately complied and climbed back to assigned altitude. The approach continued without further incident. The lowest the aircraft got to was 048 in the 052 MVA as he was rapidly descending.Recommendation: Pilots sometimes say they will do something and do something different. Not sure how to remedy this. Pilots already have high workload; but they need to be fully focused.