Horizon Focus is a terrible item though. If there was one of those Kayle_Bot 'losing to the shop keeper videos' for pre season, I'd say that item should be the first one in it.
Do the math vs Void Staff, Void will increase your damage by 10% against targets with 30 MR and obviously more than that as MR scales higher, on every ability with no conditions.
10% at 30 magic resistance
25% at 100 magic resistance
36% at 200 magic resistance
There are several champions that are strong with Horizon Focus. For example:
Ahri, Annie: Situational if you are snowballing and going for burst build.
Lux, Neeko, Lissandra, Syndra, Ori: Mages that can trigger either the Immobilize condition or the range condition easily.
Velkoz, Xerath, Ziggs, Zoe: Poke champs that can trigger the ranged condition easily.
Untested:
Seraphine: I think it's good on her due to E and R, but it might not be better than Mythic -> Rylais/Cosmic Drive -> Rylais/Cosmic Drive since she is very good with Ability Haste and the Rylais for E synergy.
Taliyah: I think this item is pretty underrated on Taliyah. It is quite easy for her to trigger the max range condition.
TF: If you are snowballing and don't want to go Zhonyas or RFC 3rd after Lich Bane, Horizon could be a good option.