I did some math to find out which AP items were the best if you wanted as much damage as possible. I only did the math for Void staff, Horizon Focus, Shadowflame and Rabadon (considering you already have Luden and sorcerer boots).
In the end Horizon Focus is always higher if you can proc its effect. So I’m wondering why every Lux player for example aren’t buying it second item.
Lolalytics show horizon as 2nd item for Lux support, both lolalytics and ugg show it as 2nd(or 3rd if you do mejai) item on Lux mid. I’d say most luxes that know what they are doing, go horizon 2nd, it’s others that blind buy shadowflame second on her no matter what. Probably same people that go full enchanter on Seraphine support rather than ap carry with rylai and mandate.