EDIT
Ahri's getting nerfed a lot in the B patch that's about to get live, and as such, I wouldn't recommend going for this comp unless it comes naturally.
Introduction
The first patch of the set was filled with Syndra and Kassadin reroll comps, and while the first has been nerfed to oblivion and the second fell from grace, a new one had gotten the spotlight. And mind you, it's been buffed in this patch.
For an obscure reason, Ahri has been buffed, as well as 4 Scholar, turning this already powerful comp into one of the top comps of the patch. Let's see it in more detail.
Late game comp (lvl 9)































What makes this comp strong
The strength of this comp lies in Ahri's Arcana bonus. For those who haven't played any game of this set yet (or who have mysteriously never encountered Arcana in their game), Arcana is a trait that will give a different bonus based on who you put the Signifier on (kinda like who you chose as the hero for Dracomancers for example).
Hecarim will give a base of 8% durability as well as AD that will scale based on the Arcana level. Tahm Kench will give HP per non-unique trait, scaling based on the Arcana level. Xerath will give the team bonus true damage per 3 charms that you bought throughout the game (even before you placed him on your board), scaling as well. A champion (or a dummy or golem) with an Arcana emblem will give the team damage amplification per emblem of any kind you have (very strong when on the emblematic golem portal), scaling too.
And the one that matters here : Ahri gives the team AP + a bonus per 3* unique champion you have, scaling of course on the Arcana level like the rest. Remember, though, that dummies and golems do not count.
Since we'll do our best to have Poppy, Lillia, Zoe and Ahri at 3 stars, we'll get a huge AP bonus, especially if we get a bonus Arcana emblem, or if we can bring in more Arcana champions. On top of that, Ahri gets 30% more AP from all sources.
Early game (lvl 4)







This is the most basic comp that you'll want, and from there, you'll focus on getting these four at the highest star level as you can. Even if you're contested by one player, you can still get them to 3*, but if there are more people to contest, it'll make finding them increasingly harder.
As per every reroll comp with 1 and 2 cost units throughout the game's history, you'll want to roll at lvl 4 or 5 (I prefer slow-rolling at lvl 5 when I can afford to, as it tends to hurt the economy much less). When you get at least two of the three 1-cost units at star level 3, push to level 6 to be able to get Hecarim in and increase your odds to hit Ahri 3. When she is, stabilize your economy and aim for level 8 to bring in 4 Bastion with Taric, and 4 Scholar with Ryze and Bard. You'll also get Portal on at the same time, as a side bonus.
Before having hit Poppy, Lillia or Zoe 3 (preferrably two among them), don't bother buying units of a higher cost such as Ryze or Taric as you won't be able to put them on your board until much later, and it'll cost both gold and space on your bench.
When NOT to aim for this comp
If you start with mostly AD items, don't even consider this comp, as you'll struggle a lot to get the specific items you need such as Blue Buff and Guinsoo for your Zoe. Picking Pandora's Items to compensate would be a waste, as you can't know when you'll finally get your desired items, while you could have a combat augment.
Also, if you only have one copy of each unit of the early comp by the end of stage 2, don't waste your time forcing this comp, you'll most likely just bleed out and struggle too much to match the pace of the game.
Good augments to play with this comp













Spellblade might be the best augment to start with this when you can consider this comp due to having already gotten several Poppy, Lillia and Zoe, even if it's been nerfed. And it might be better to consider forsaking 4 Scholar in favor to a higher Arcana level to benefit from a better AP bonus.
Conclusion
This comp requiring early rolls, and often being contested, it's not one that will assure you a top, but if you have a favorable early game, it can really snowball hard.
I'll make another guide for the variant that is also doing quite good : Ahri Vanguard.
If you've got any suggestion to improve this guide, feel free to comment, and to follow if you want to easily access any previous or future guide.