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.
It depends on champions, but it is a good item.
Gragas for example tends to build it over Shadowflame, it’s usually on champions who can proc horizon consistenly with their kit, but I see it built more often these days.