I think you are referring to the destruction of the collection of the State Archives of Naples on 30 September, 1943, the Grande Archivio di Stato Napoli. I hope so, because I'm very glad you asked about it. It is very poorly known about and was an extremely important and tragic event, one for which the Germans never properly answered. The contents of the Archive had been packed up in over 800 crates and had been moved to the Villa Montesano, near San Paolo Belsito, in the Nola region, about 30 kilometers from Napoli. It was over 30,000 manuscript volumes and 50,000 documents. By late September 1943, the Allies were drawing near, and German scavenger/rigger crews of three to five men had been fanning out across the countryside, looting everything they could and rigging everything else with mines and booby traps. Many many many historical buildings were destroyed in the next few weeks. When they came to Montesano on 28 September, they forced their way in to the villa and discovered both the many piles of crates there and also the remnants of the Archive staff, which were there to look after the contents. The Germans asked what it was, cracked open a few of the crates, and were told the contents were of importance to Italian cultural heritage. They went away but soon a different group and an officer appeared, again forcefully entered, and again went away. There was a sense of foreboding, especially after a German soldier was killed in the nearby town. Count Riccardo Filangieri was nearby and although he says he was not initially alarmed, since up to that point the Germans had not embarked on any deliberate campaign of cultural heritage destruction, still he thought it prudent to write a letter to the local German commander. In the letter he explained that the documents belonged to the Naples Archive, that they were of great cultural importance not only to Italy but to all of Europe, including German history.

This was too late. A squad of Germans arrived on the 30th in the morning and announced that the entire archive would be destroyed. Attempts to communicate with the German officer were in vain, though the Count says his letter was shown and even translated into French (I guess they could not translate it to German). He reacted angrily and within minutes the entire villa was in flames, along with all the contents. Evelyn Jamison wrote that "The deliberate, senseless destruction of the documents belonging to the Grande Archivio di Stato of Naples is one of the greatest, if not the greatest, single crime against history committed by the German army during the late war." In addition to the tens of thousands of precious documents, the villa also contained the contents of the Count's own art collection, including pieces by Van Eyck, Boticelli, Chardin, and Pontormo, as well as his personal collection of ancient ceramics and glasswares.

The Count tells us directly about these events in "Report on the Destruction by the Germans, September 30, 1943, of the Depository of Priceless Historical Records of the Naples State Archives," American Archivist Vol. 7, No. 4 (Oct., 1944), pp. 252-255. He writes:

The extent of the disaster is enormous. In that depository I had collected all the most precious series of documents coming from the various archives of the south of Italy. And their destruction has created an immense void in the historical sources of European civilization, a void which nothing will ever be able to fill.

He lists what was destroyed. All of this material is early medieval or later, as far as I can tell, but it would be great if some Medievalists in the sub could offer further comment about what some of these collections contained specifically. I am supplementing the count's terse list with some info from Evelyn M. Jamison, "Documents from the Angevin Registers of Naples: Charles I", Papers of the British School at Rome Vol 17 (1949), pp. 87-173.

• Registri of the Anjou Chancery (1265-1435); Jamison calls it 507 volumes total, with the 378 Stately Registri on parchment and bound in leather; 66 Fascicoli of administrative acts on paper from the 13th and 14th centuries; the 38 volumes of "Arche in pergamena" and 21 volumes of "Arche in carta." Together they were "a veritable storehouse" of history on the Kingdom of Sicily, and "every aspect of its life." Jamison reports that what was lost in this has (or will be) partially restored via notes and small extracts preserved elsewhere (so some good news!).

• the registers of the Aragon Chancery

• the manuscripts, codices collections, famous political trial transcripts, and original treaties of the kingdom of Naples

• the greater part of the official archives of the Royal House of Bourbon

• most of the archives of House Farnese (some were saved from the fire)

• documents of the Royal Summary Court and Collateral Council

• ... of the Royal Chamber of Santa Chiara

• ... of the Secretariat of the Viceroy

• ... of the Superior Chaplaincy

• ... of the Council of Sicily

• ... of the Ministries of the Presidency and of Foreign Affairs of Bourbon

• ... of the Order of Malta

• ... of the Feudal Commission

• ... of the "Ancient Treasury"

• ... of the Conservatory Court of the Nobility

• "of the most ancient notaries"