Ahri has been seeing more plays by high-rank players in Patch 11.5 and 11.4.
Used to be one of the best picks of League in Season 10, Ahri was a popular choice in mid-lane with builds that revolve around Glacial Augment. However, the complete overhaul of the item system in Season 11 has not been favoring this Nine Tails Fox so far.
It was just until Patch 11.4 with the massive change for Everfrost that Ahri made her comeback in Mid. Everforst allows her to clear wave much better, while also provide an extra cc and a decent ability haste stat to help her cooldown Ultimate much faster.
Cosmic Drive new Passive, which was also release on Patch 11.4 also synegize really well with Ahri, as it provides extra cooldown and movement speed (active upon having 160 AP or more)
Everfrost, Cosmic Drive, Ionian Boots of Lucidity, and running Transcendence in her Rune page will help the Nine-Tails Fox reach 100 Ability Haste at mid-game (about 50 percent cooldown).
To further improve her damage, Horizon Focus is the best pick. Combine with Electrocute as her main Keystone, she can have a really decent amount of damage in combats if she can land her E perfectly.
Zhonya’s Hourglass or Banshee’s Veil can be really good situational items if you need extra protection against certain matchups.
With this build, Ahri will become a high-mobility threat to ADCs and AP carries as the game drag more to the late game, with strong engagement skill, she can play really actively and start teamfights effectively.