Complex sentence: He returned during Henry's investigation into retrieved Nazi art to try and find a lost dagger that he had owned before the war, but although he failed to find it, Adam provided Abe with a ledger that allowed him to find information about his biological parents.
Split into simpler sentences: He returned during Henry's investigation into retrieved Nazi art to try and find a lost dagger that he had owned before the war. Although he failed to find it, Adam provided Abe with a ledger that allowed him to find information about his biological parents.