How do you rotate camera down 45 degrees no matter the direction?

Here is my code, I've been working on this for hours, I just can't get my head around it.

    // when user hits c key, the camera points down 45 degrees towards terrain
     if (Input.GetKey(KeyCode.C))
     { 

         //Quaternion wtf = transform.rotation;
         transform.eulerAngles = new Vector3(1, 0, 0);
         //float tiltAroundZ = Input.GetAxis("Horizontal") * 135;
         //float tiltAroundX = Input.GetAxis("Vertical") * 135;
         //Quaternion target = Quaternion.Euler(-tiltAroundX, 0, tiltAroundZ);
         //transform.rotation = Quaternion.Slerp(transform.rotation, target, Time.deltaTime * 1.0f);
         // Get a copy of your forward vector
         Vector3 forward = transform.forward;
         // Zero out the y component of your forward vector to only get the direction in the X,Z plane
         //forward.y = 0;
         //float headingAngle = Quaternion.LookRotation(forward).eulerAngles.y;
         //transform.rotation = Quaternion.LookRotation()
         transform.rotation = Quaternion.Euler(45, transform.rotation.y, transform.rotation.z);
         //Quaternion testing = new Quaternion(0.5f, 0, 0, 0);
         //testing.x = 0.5f;
         //transform.rotation = wtf * testing;
         //transform.rotation = new Quaternion(0.5f, camRotationY, 0, 0);
         //transform.Rotate(new Vector3(0.5f, 0, 0));
         //and if (Mathf.Round(transform.eulerAngles.z) == 180) { transform.Rotate(new Vector3(0, 0, -180)); }
         //transform.rotation = testing;//Quaternion.Slerp(transform.rotation, transform.rotation + Vector3.down, 2.0f);
         //camRotationX += testing.x * Time.deltaTime;
     }