An instrument of surrender is a surrendering document of a military conflict, as those documents are legal instruments. Some such documents are:
World War II Japanese Instrument of Surrender German Instrument of Surrender Armistice of Cassibile Armistice of Malta
Other conflicts and uses Argentine surrender in the Falklands War 1982 Pakistani Instrument of Surrender (1971) at the end of the Bangladesh Liberation War "Instrument of Surrender" from the 2019 Disco Elysium (soundtrack) by Sea Power