Short answers, pulled from the story.
Karl Dönitz was a German grand admiral who served as Supreme Commander of the Kriegsmarine U-boat arm and, from January 1943, Commander-in-Chief of the entire German Navy. He was the principal architect of Germany's submarine campaign in the Battle of the Atlantic and became head of state of Germany following Adolf Hitler's suicide on the 30th of April 1945.
The wolfpack, known in German as Rudeltaktik, was a tactic where multiple submarines operated together as a group to overwhelm the escorts of Allied merchant convoys. Dönitz revived the concept from Hermann Bauer and refined it using ultra-high-frequency radio transmitters and the Enigma cipher machine for secure coordination. Submarines attacked on the surface at night, where early sonar technology known as ASDIC could not detect them.
Around 30,000 of the 40,000 men who served in U-boats perished during the war. Of 648 U-boats lost, 429 were sunk with no survivors. Two hundred and fifteen boats were lost on their very first patrol.
Black May refers to May 1943, when Allied air and naval forces inflicted catastrophic losses on the U-boat fleet. Dönitz withdrew surviving crews from the mid-Atlantic on the 24th of May after losing 33 U-boats before the withdrawal, with the total reaching 41 by month's end. The combination of escort carriers, support groups, and Very Long Range aircraft closed the mid-Atlantic gap that had sheltered the wolfpacks, ending Germany's ability to mount effective convoy attacks.
Dönitz was indicted at Nuremberg on three counts: conspiracy to commit crimes against peace, war crimes, and crimes against humanity. He was found not guilty on the charge of crimes against humanity but guilty on the other two counts. The court sentenced him to ten years' imprisonment.
The capture of the German meteorological vessel München on the 7th of May 1941 and the submarine U-110 two days later gave the Royal Navy Enigma machine settings that allowed decryption of U-boat communications. Beginning in August 1941, Bletchley Park could read signals between Dönitz and his submarines without restriction, enabling the Admiralty to route convoys around wolfpacks and identify individual boats, their commanders, and their positions. Germany introduced the M4 cipher machine on the 1st of February 1942 to restore security, but it too was cracked by December 1942.