As @Shyju said you should use Content
method, But there's another way by creating a custom action result, Your custom action-result could look like this::
public class MyActionResult : ActionResult
{
private readonly string _content;
public MyActionResult(string content)
{
_content = content;
}
public override void ExecuteResult(ControllerContext context)
{
context.HttpContext.Response.Write(_content);
}
}
Then you can use it, this way:
public ActionResult About()
{
ViewBag.Message = "Your application description page.";
return new MyActionResult("content");
}