From Mud and Blood official Wiki
A multi-purpose cannon used for supporting infantry and ripping you a new one. It only appears in the form of an unfair event. When this unfair event is triggered, three arty sized shots will be fired rapidly, and will land randomly on the map. Be careful, it can bust your bunkers and destroy your other constructions and vehicles.
The Pak-88 ingame was actually called the Flak 18, or the 88 mm gun (eighty-eight) in World War II, and was one of the most famous artillery pieces during the war. It was used both as a traditional artillery piece, an AAA gun and an AT gun. It was one of the best artillery guns of the war, as it fired very rapidly. Flak is a German contraction of Flugabwehr-Kanone. Over 9 000 of these in different models were produced during the war. They were used in all fronts as artillery or AAA, but on the Soviet Front it was their main AT-gun as the T-34 and KV1 had too thick armor to be penetrated by other AT-guns. They were also used in the African Front where Rommel lured British tanks to a trap where Flak 88's started firing at them from so far away that they couldn't return fire. Eventually British heavy artillery and land forces began to work together and bombard 88 positions after they revealed their location. With the arrival of the 75mm Sherman, the allies could fire at about the same range as the 88 could shoot at the Sherman.