Hello everyone. I'm Eden, a diamond botlane (currently). I play both role ADC and support. I played LoL from 2017 - now. I very like Nilah since her release and became Nilah mains after a few month. She is very fun to play but not too popular. So I want to share some Nilah guides from me.
And let u understand her too :>

She is a melee ADC that suppose to be botlane. She has high burst damage and can evade normal attack from other ADC. She can scale very hard, make she becomes a monster in long game. If you want a champion that go botlane but not traditional ADC, then Nilah is for you.

Pros:

• High burst damage.
• Have 2E for mobility.
• Can evade basic attack and reduce magic damage.
• Strong healing and high armor penetration.
• Can 1vs1 many champion.
• High scale.
• Can 1vs2/3/4/5 when ahead.

Cons:

• Low range, being poked is nightmare.
• She is melee but her stat is the same as range champion (low AD, low armor, low health).
• Without W, she is vulnerable.
• No E = walking turret, high damage but easy to kite.
• Her E has short range, and a little bit clunky delay.

- Priority: Xayah, Caitlyn, Brand, Veigar. Their range and skill are annoying.
(This is only for botlane matchup. If you scare any champion, as your wish. But better ban those for better gameplay <3).
- If you meet a hard counter team (like enemy have Syndra, Xayah, Taliyah,...), better dodge that game. If there are many basic attack champion like Yasuo, Yone, Yi, pick Nilah XD
- Only pick Nilah when you know enemy botlane and the other don't counter you too much. (Pick Nilah in a team like Aatrox, Taliyah, Syndra, Xayah, Alistar is nightmare.)

P: JOY UNENDING

- Whenever Nilah kills a minion, she and the nearest allied champion gain an additional 50% of the experience they would have lost from sharing.

(That means Nilah and the nearest ally will gain 75% total XP from sharing instead of 50% for both. But Nilah must be the one who takesdown minions.)

- Whenever a nearby allied champion uses an ability to heal or shield either Nilah or themselves, both they and Nilah receive a bonus amount equal to 7.5% of the healing or 15% of the shielding after a 0.5-second delay. Bonus shielding lasts as long as the shield that triggered the effect, up to a maximum duration of 4 seconds.

(That means if Nilah got heal (shield) from champion allies, she will heal for 7.5% more of that heal (15% for shield). If any allies heal (shield) themself, Nilah will heal (shield) herself for 7.5% of that healing (15% for shield).)

Note: The heal/shield from allies item like Redemption, Locket of the Iron Solari is not counted for her passive. However, lifesteal and jungle pet heal from allies are counted.

Q: FORMLESS BLADE

PASSIVE:
- Nilah gains 3.3% armor penetration per 10% critical strike rate, up to 33% armor penetration.
- Basic attack and Formless Blade will heal Nilah for 0% - 20% based on her damage. Heal instance beyond maximum health will convert to a shield that lasts for 4 seconds.

ACTIVE:
- Nilah cracks her whip-blade in a line in the target direction that deals 5/10/15/20/25 (+90%/95%/100%/105%/110% total AD) physical damage to enemies hit. This damage will increase by 0 - 100% (based on critical strike rate). This active can hit structure and apply lifesteal.
- If Formless Blade is cast during Slipstream, Nilah leaves behind a wave that is sent towards her location at the end of the dash, dealing the same damage to enemies hit.
- If Formless Blade hits an enemy, Nilah empowers herself for 4 seconds: gaining 125 bonus attack range and 10% − 60% (based on level) bonus attack speed, and causing her basic attacks to strike in a cone that deals 100% AD physical damage to enemies hit. Against secondary non-champion targets, this is reduced to 33% AD for minions and 10% AD for monsters, and additionally executes minions left below 20 health. On-hit effects are applied only to the primary target.

(- This is Nilah main skill, where she strikes enemy in a line like Yasuo, and Yone, then she empowered herself. This skill has casting time and can be reduced by attack speed (like Yasuo Q, Yone Q, ...). Using Q when dashing with E will damage all enemy on the way and a little distance at the end of the dash.)
- About her passive, with LDR/MR, she can gain about 50% - 60% armor penetration. The healing from passive is not counted as lifesteal. But her Q and the shield can benefit from lifesteal.)

W: Jubiliant Veil

ACTIVE:
- Nilah envelops herself in mist for 2.25 seconds, during which she becomes ghosted, gains 15%/17.5%/20%/22.5%/25% bonus movement speed, dodge all non-turret basic attack and reduce magic damage taken by 25%.
- If she touches any allies champion during that, she will gain them the same effect for 1.5 seconds.

(This skill is like Jax E, but doesn't have stun like him. In botlane where basic attacks are mainly used, her W is completely broken. Not to mention that she can share the effect with her allies. Also, skill that is classified as basic attack is also blocked, like Ezreal Q.)

E: Slipstream

ACTIVE:
- Nilah dashes a fixed distance in the direction of the target unit, dealing 65/90/115/140/165 (+ 20% total AD) physical damage to enemies she passes through.
- This skill can have up to 2 charges.

(This skill is surely 100% copy from Qiyana E and Samira E. However, there is a small delay when use this skill, make it look a little bit awful. This skill can dash though wall, but sometimes it will be blocked even her max range can pass though terrain. This skill can reset basic attack timer like Ashe Q, Vayne Q. Moreover, she can use Q and R during her dash, but not W (weird LOL).)

R: Apotheosis

ACTIVE:
- ACTIVE: Nilah whirls her whip-blade over 1 second, dealing 15/30/45 (+20% bonus AD) physical damage to nearby enemies every 0.25 seconds. She then unleashes a burst that deals 125/225/325 (+ 120% bonus AD) physical damage and pulls them 250 units towards her. Each hit also slows targets by 10% for 3 seconds, refreshing on subsequent hits.
- Nilah heals herself and nearby allied champions for 20% − 50% (based on critical strike chance) of the damage dealt to champions, reduced to 10% against non-champions, converting each heal instance beyond maximum health into a shield that last 6 seconds.

(When using R, she can't use any skill or basic attack besides her W and spells. This skill not only deals damage but also heals herself and allies and provide some CC. Spell shield likes Sivir E only can block the burst after 1 second.)

- Early game:

+ Nilah is easy to abuse at level 1. She has no mobility, no range, no defensive skill. Try to farm safe at level 1. If enemies don't harass too hard at level 1 and you can level 2 first, try to all in with your support. You may take around 1 to 3 spells from them. If your level 2 is fail, wait for level 3 and all in again. If you meet hard counter or/and strong harass, farm safe and wait for gank from jungle or midlane.
+ Remember this: Nilah and her support get level 2 when takedown first 8 minions (9 for "non-Nilah" champion. They get level 3 when clearing half of the first cannon minion waves.)

- Mid game:

+ At this time, you or enemy might have the turret (but mostly enemy duo to Nilah range). Swap to midlane (or sleep in botlane :> for the turret) and try to join every combat (But still need to farm minions, don't go around like a fool). In lower elo, midlaner may not swap for you, so you and your support just sleep in botlane and takedown several turret + dragon.
+ When you have at least 2 items, you are gonna slap enemy in their face. At this, Nilah is strong enough that can slap enemy and turret like a baby. (But make sure your team don't feed LOL, or u r gonna cry)

- Late game:

+ This is where she scales so strong that if she is ahead, she can even get a penta, and even 1vs2/3/4/5 or 2vs3/4/5 with your support. (But if both team are having equal items, combat carefully LOL)
+ At this time, you are likely an assasin. Focus on enemy midlaner or ADC. Using R + flash can wipe out them very fast before they can react.

- Water Dragon soul is the best soul for Nilah because she can increase healing from Dragon soul for her teammate (but not herself bruh).
- She can gain a support from takedown if any ally heal or shield for themselves in Nilah range.
- Her E can dash on ally, but not turret or ward. When use Q with E, her Q cooldown is not benefit from AH (bug?). I don't recommend use E+Q because it isn't stable, and easy to dodge if enemy is too mobility.
- Champion with repeated heal like Swain, Pyke, Milio will trigger Nilah passive rapidly, make it sound very annoy. (:>)

Nilah is a fun champion to play. The lower elo, the more free LP you gain from her. Understand her is not too easy, but when you master her, you will feel the joy, and how broken she is.
Joy, unceasing and forever.
Sorry my english is bad and my guide may not good 'cause it's my opinion. Some people may have better guide. Don't judge this too much, just a little guide for reference.
Also I don't post my ingame account here, please refer to Enryu for better information.