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.
because mages don’t really care about DPS too much
if they get caught on having their spell up in 5 seconds or 4 doesn’t matter when they die in 2
if they don’t get engaged on they can just back off and wait for another rotation
so they either do that or try to oneshot you,and shadowflane is slightly better at that