I recently played a bunch of Xayah in ARAM and was messing around with her runes and items to see what was most effective. Came upon Ravenous Hydra in one of my matches and ended up getting 30 kills, so I continued to mess with the build and ended up coming out with this build. So far in my test games I've had some pretty widespread success so I think it's worth turning into a guide for those who like Xayah Mid or want to try it out. I Ended up with a 10 game winstreak of me winning 90% of my lanes and beating some tough teams and surprisingly beaten some usually difficult champions like Akshan and Qiyana.

Insane Wave Clear: With this build you have some of the best wave clear in the game PERIOD. You throw 3 feathers into the back of the wave and delete it with ease. Nobody can shove you back. It also means it's very easy to collect gold and cs like candy.

Solid Early Damage: Your early damage with this build is surprisingly great with greaves rush+duskblade and make it so you can trade with most matchups in mid lane.

Very Few Mana Issues: Since we're running Presence Of Mind and Essence Reaver there is almost zero times where you'll be running out of mana often or even just struggling to manage your mana pool.

Great All-In/Assassins Counter: Xayah's kit in general is just really good for countering all-in champions and aggressive gankers. Her solo lane potential because of her R alone is amazing and should be more popular imo.

Squishy: Despite being a counter to all-ins she's still mainly an ADC so she's still really squishy and quick to death if not positioned properly in a fight.

No Mobility: Xayah has no mobility in her kit, and not having a gap closer can be fairly challenging in chase scenarios and early ganks. Best way to avoid getting gapped by your lack of mobility is to just last hit and let the waves come to you instead of pushing up to the wave.

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  Press The Attack is a solid keystone on Xayah because she's good at weaving autos between her feathers and after hitting the third stack of it on a champion, you deal bonus 8-12% bonus damage to them for 6 seconds. This means you can hit the 3 stacks, then E to pull the feathers and deal an extra chunk of damage every time you pop the third stack.
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  Presence Of Mind will be our main mana retention until we get Essence Reaver, and should provide enough help managing your mana pool to be comfortable early.
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  Alacrity will give us some added attack speed to compensate for the lack of attack speed in the build itself. It scales up as the game progresses so while it isn't as effective early game, it'll come full circle once we're comfortable and prepared for the worst.
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  Last Stand will be great for this build because there are a lot of times where you might get low in a fight or get ganked and have a sizeable amount of feathers built up. With Last Stand the lower you are the more damage you do, so if you're close to dead you deal around 11% bonus damage and it makes those feathers a lot deadlier as a whole.
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  Cheap Shot adds some extra true damage after impairing enemy champions' movements. Meaning we can E root and then on the next attack or ability deal bonus true damage to go along with all the other damage buffs we get from our runes.
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  Ultimate Hunter is one of the more important runes for this build because Xayah's ultimate is what makes her solo lane so good. Her ultimate makes certain matchups completely obsolete and stops aggressive junglers from getting free dives. One example is Vex. If Vex tries to Ult you all you have to do is Ult as she's traveling towards you and it not only deals no damage but there's a good chance they're also going to blow their fear. Now you just E and pelt her to death because she can't do anything to beat you. So you'll want her Ult up as much as possible.

Note: Since this is a Xayah MID build I won't be including information about Rakan interactions to try and condense all the info a little better. Just know if you're going ADC Xayah that she has certain interactions with Rakan in her kit.

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  Innate: After casting an ability, Xayah's next 3 attacks launch feathers feathers that go through all targets except towers and other inanimate objectives. The feather deals full damage to the initial target and reduced damage to all enemies hit after. The reduced damage can still critically strike. After the attack the feather will be left on the ground for 6 seconds. Xayah can store 5 feathers for 8 seconds, freshing with ability use or attacking structures.
  Note: Once the feathers are left on the ground, an arrow can be seen directly under the feathers. This shows the path the feathers will take back to you if or when you use Bladecaller. You can use these arrows to know if recalling the feathers will hit the enemy you're aiming for or not.
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  Active: Xayah throws two feathers, dealing physical damage to all enemies hit, and leaving 2 feathers on the ground. targets hit after the initial target take 50% reduced damage.
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  Active: Xayah creates a swirl of blades for 4 seconds, granting her bonus attack speed and adding an additional attack to your autos that deal 20% damage. This additional attack works on any enemy or structure in the game. If the additional attack strikes an enemy champion, Xayah gains 30% bonus movement speed for 1.5 seconds.
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  Active: Xayah recalls all feathers currently on the ground, dealing physical damage to all enemies they pass through and dealing increased damage based on your critical strike chance. Bladecaller can only hit enemies once per feather and any enemy hit by 3 or more feathers will be rooted in place for 1.25 seconds.
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  Active: xayah leaps into the air, becoming untargetable and ghosted for 1.5 seconds. Near the end of her leap she shoots out 5 daggers in a cone, dealing physical damage and leaving behind a feather for each dagger. During her leap Xayah cannot use any attacks or abilities but can still move and can still use spells like Flash to reposition.

So the reason we're rushing Berserker's Greaves first item is because attack speed is really good on Xayah and makes it easier to setup feathers behind enemies and clear waves. The early attack speed also helps with Press The Attack and early trades until you can get your Duskblade or even just Serrated Dirk. It's also really good for side-stepping skillshots and getting out of large AOE abilities that would otherwise be difficuly to get out of.

Duskblade of Draktharr is a solid item on Xayah as she uses Lethality items well and it gives 2 passives which both work wonders on her. The first passive is the passive that deals bonus damage on the first auto attack that hits an enemy champion which fits with our runes and other items to deal tons of damage immediately and not give the enemies a chance to retaliate. The other, and main passive that we'll be utilizing the most is the passive that gives us invisibility whenever we score a takedown on an enemy. Meaning you can go in with R, E for a kill, and then reposition with your invisibility and go for another attack. You can repeat this process until the fight is over and come out essentially untouched as long as you're not focus number 1 in the teamfight.

Essence Reaver will be our main mana retention item and will also give us a sheen to partially abuse. It's fairly cheap and will be a solid transition item into our other expensive item so we dont get bogged down trying to collect a million gold in the mid game. It also gives us some crit which is nice with Xayah's E having a crit chance modifier on the damage.

Revnous Hydra, despite being a pretty expensive purchase, is one of my favorite items on Xayah. The reason why Ravenous Hydra works so well on Xayah is because her feathers from Q, passive, E recall, and R active all activate Ravenous Hydras passive which deals 60% physical damage in a small zone around the initial impact of the feathers. so you can auto+Q for 3 feathers through wave, then E back and the damage of your feathers+Ravenous passive will delete the wave entirely. It's some of the best wave clear in the game and makes farming through the mid and late game easy. It also gives the most base AD in the game next to Infinity Edge at 70.

Lord Dominik's Regards is a great late game item for Xayah as it hurts tanks the most while still adding damage and crit to further demolish squishies. The big part is the 5-15% bonus physical damage to enemies with more max health than you, and since we're not getting any health in our build, any tank we come across should take 15% bonus physical damage.

Runaan's Hurricane will be our final item in the main build and another attack speed item to make sure we're up to standard for late game. It's also a cheap item and the arrows that branch off also trigger Ravenous Hydra passive for a little bonus damage. This Item is great for teamfights and splitting aggro to better transition to other targets after the first is dead.

Guardian Angel is best for when you're the carry and you know for a fact that without you the entire team just collapses into dust. It's also a decently cheap item and gives some added armor against high AD comps. If you go downw with Guardian Angel active, make sure to spam ping assist because im sure if you're buying this item you've got meatheads for a team. Edge Of Night is for when you need a spell shield but you still want to get some damage output. The one downside of this item is that it slightly clashes with Lord Dominik's Regard by giving 325 health. Still, I'm sure it's better to have the spell shield and live rather than do slightly more damage and die every teamfight. Finally, Mortal Reminder is a good option when you want crit and the enemy team has tons of healing. It's also going to help with your movement speed and will be good for kiting late game.

Early game will mostly be spent farming and avoiding death. I usually try not to be too aggressive because falling behind can be extremely difficult to come back from into some matchups. If you can, do some small trades to wear the enemy down and keep them from attempting an all-in. One big thing to do before level 6 is avoid pushing up. Even though your E can root someone, if you get engaged on and CCd before you're able to get enough feathers to root them with you just die.

Mid Game is where you start building up your damage. Keep farming as much as possible but keep your eye out for objectives and skirmishes near mid lane incase you have to rotate to help your team. The reason we're farming so much is because duskblade and ravenous hydra are costly items and if you can get them earlier in the game it'll make a world of a difference.

Late game is where you go full kill hobo and wipe the floor with the enemy team. You can essentially one shot any squishy with 4-5 feathers and you kite well enough to shred tanks effectively. Wave clear isn't an issue as you have wave clear that is unrivaled and outclasses anyone in the game. The only thing to really focus on now is objectives and not dying. Keep farming, keep getting kills, keep getting items.

1.
Vex is the easiest matchup for Xayah hands down. Her early damage can be a bit annoying yes, but since we're rushing greaves it'll be easier to side step and avoid her abilities. She has no mobility so a well placed feather sequence will pretty much be unavoidable. The big kicker is level 6. From level 6 on save your Ultimate until she uses her Ultimate. If she hits the ultimate, wait until she starts traveling to you to use your Ult. By the time she gets there you'll be untargetable so not only will she do no damage, but she'll be directly in front of your R and most likely blew her fear spamming keys. Now you just land your R+E, and fill her with feather blades while she has nothing to kill you with. It's literally a free lane if you know what to do.
2.
Kassadin is another easy matchup. During lane phase you just harrass him, outfarm him, and then once 6 hits if he goes for an all-in just R+E and he'll either be forced to back off or he'll be dead. If possible always try to keep a few feathers in the back of a wave just to root him if he goes for another all-in. Level 2-5 though you can bully him as long as you keep your distance.
3.
Syndra is a strange but good matchup because similar to vex, us rushing Greaves first item helps us side-step Syndras' abilities while still being able to trade. She has no mobility and unlike Vex, she has no way to mitigate damage so a well placed R+E will shred her. The one thing to be wary of is her Ultimate. It's point and click so there's no way to dodge it. The one thing you can try is using your Ult to cancel hers out, but I haven't fully tested whether Xayah Ult blocks Syndra Ult or if Syndra Ult just goes through anyway, so take that with a grain of salt.

This build is one I found out of pure coincidence and has risen to be a really fun alternative to standard xayah. I've had near perfect success with this build so far and don't see that changing in the near future. Legit one of my favorite off-meta picks at the moment. Hopefully you find the same success I did in your games, GLHF!