return RedirectToAction("Index","Auth");

return Json(new{data=true})

$.ajax({
            type: 'POST',
            url: 'your url',
            data:your data
            success: function (data) {
                if (data) {
                    window.location.href="url";
                }
                else {
                     window.location.href="url";
                }
            },
            error: function (xhr, textStatus, errorThrown) {
                alert(errorThrown);
            }
        });
        return false;

window.location = '@Url.Action("Index","Auth")';