When quinine was used as a treatment during a malaria outbreak, a small propoion of patients developed blackwater fever i.e. chronic kidney damage from the autoimmune effects of complement mediated hemolysis of quininized RBCs. In this case, what was the role of quinine?

A hapten is an isolated (free) antigenic determinant, too small (by definition) to trigger an immune response on its own. Quinine binds to the surface of the infected and uninfected RBCs and is recognized in association with RBC surface antigens. Therefore, it is serving as a hapten, the RBCs are the carrier, and together they are an immunogen against which the immune response occurs.