
Free Agent - NA
Intro: This is just how I play Nyx. I like her a lot and I'm pretty darn good with her.
I will say, Nyx is INCREDIBLY team reliant. By that I mean she needs the support of her team to reach her full potential. With help from the damages and supports, she can STEAMROLL an enemy team. She's best at giving supporting damage/CC, not so much securing kills. I'm not the most experienced with Off-Tank Nyx, but I tried to give ya a guide. That's not my forte XD. I definitely feel more secure in my Point Tank advice.
SUPER IMPORTANT HAVEN NOTE: (Thanks u/BartOseku on reddit!)
I JUST learned this. When combined with Royal Presence, Haven activates TWICE.
Royal Presence applies DOT to Nyx. When you have Haven (or any DR), Haven is activated when the DMG is dealt to Royal Presence, and then AGAIN when Royal Presence deals the DMG to Nyx.
Here's an example scenario:
Nyx has 10% DR up. Nyx also has Royal Presence active.
Someone attacks Nyx for 100 DMG.
Royal Presence absorbs the DMG
DR activates, so Royal Presence absorbs 90 DMG
Royal Presence deals the DOT to Nyx.
DR activates AGAIN.
Nyx is dealt 81 DMG.
Because of this, Haven is INSANE on Nyx. That's why she can live so long. With bigger numbers, you're reducing a LOT of DMG with Royal Presence, and even without it, you'll still have 18% DR with her massive healthpool.
Ability Notes: Nyx is similar to Inara in the sense that you need to time out and rotate your abilities.
Rift Slash should be spammed as much as possible. Having that CC and Damage on the field at all times is incredibly helpful. Also, this way you're being more efficient with the cooldown reduction card. If there's no Rift Slash cooldown to reduce, that card is useless. So use it.
After that, Royal Presence should be up. This lets you get damage out as well as preventing damage on yourself. Just keep in mind that Nyx is decently slow and has no mobility. You need to be proactive with your retreat/shield. The Chain Pull isn't worth ending the ability early 99% of the time. Everyone once in a while I'll give someone a yank, but for the most part I just let the ability run out. The pull is difficult to target and so weak it's not worth it.
Abyssal Fortress should really only be used as close to you as possible. Occasionally I'll shove a shield out to help a squishy, but that's only if my own position is secure. Use it once your health is low or if you have a lot of damage absorbed with Royal Presence. Be proactive so you can cleanse caut and get healed up faster.
Her ULT is a great counter-ult. Put up Royal Presence, absorb damage, then delete it and punish those nearby. Keep in mind the ULT affects enemies above you and through walls. You can use it to pull someone off of a vantage point or attack them from below.
Her Passive is amazing as a Point Tank. Bonus healing and movement speed are NOT subject to diminishing returns, so Rejuv is fantastic on top of her Passive. Since she starts out with 20% Rejuv, you don't need to buy it early game, but it should still be a priority. I'd say make it your second item pick in most cases.
Cards:
The Little Things - Reduces the cooldown of Rift Slash with every Inhand Weapon strike. This is one of her best cards because Rift Slash your only method of "burst" damage, if you could even call it that, and gives fantastic CC.
- Hitting shields/deployables activates this card
- The cooldown is reduced even while the Rift is still active. It just doesn't show it on the icon.
- It's not really worth having this card above 4. Keep in mind that AS you're punching the cooldown is going down as well, and there is a maximum amount of time you can take away. As long as you hit your shots reliably, you don't need it too high. And aiming with Nyx isn't too difficult.
- Chronos doesn't affect this card negatively, since it's a set number and not a percent. So that's good!
True Freedom - Lets you miss Inhand strikes without resetting the combo. This card is GREAT for beginners. When you're learning Nyx, I recommend always having it at Level 1 in a build. However, once you're hitting shots reliably don't forget to swap it for something else.
Center of Combat - Increase the percent of health Royal Presence damages. I think this card is either all or nothing. Have it at 5 or don't have it at all. At 5 it's great, at 1 it's negligible, and anything in between is meh. If you're gonna use Royal Presence to deal damage, go full throttle.
Unyielding Advance - Increase the health of Abyssal Fortress. Her shield at base is really lackluster, so you wanna buff it, right? But honestly it's more worth it to go Forever Changed (increase base health). If the enemy goes wrecker, this card is pretty much still useless. And it only works when you have your shield up. Forever Changed gives the same effect of more survivability while being more difficult to counter AND active at all times. Plus, Forever Changed can make Veteran more effective if you want to use it.
This build focuses on Nyx's CC and Cooldown Reduction.
Just follow the basic playstyle I listed in the intro. Abyssal Authority can easily be replace with whatever you might want to. I just think the extra range can help affect as many enemies as possible.
Her best talent is by far Show of Force. Resil can remove the slow, but it can't remove the cripple. You can use Rift Slash to prevent enemies' escapes and zone choke points a bit.
In my experience, starting with Haven and Rejuv is insane. Get Haven 1, then Rejuv 1, then go for Haven 3. After that, focus on Rejuv and Kill to Heal. Nyx's Passive means you don't NEED to put a lot of resources into Rejuv early game. But you still want it sooner rather than later.
You can use Subjugation instead of Show of Force with this build if you think there's gonna be a hypermobile champ causing problems. But with good timing Show of Force can decently accomplish the same thing.

Show Of Force
Rift Slash Cripples enemies it hits for 2s, and the resulting Abyssal Rift lasts 1s longer.
Level 1

Unchecked Power
Rift Slash
Increase the slow strength of the rift's pulses by 20%.
Level 4

All Will Kneel
Rift Slash
Increase the Abyssal Rift's pulse area of effect by 48%.
Level 4

The Little Things
Reduce the Cooldown of Rift Slash by 0.8s, up to a max reduction of 5s, when hitting enemies with Realm Breaker.
Level 4

Abyssal Authority
Increase the radius of Royal Presence's Poison aura by 20%.
Level 1

Forever Changed
Increase your maximum Health by 300.
Level 2

Haven
Armor
Reduce the Damage you take from Direct and Area of Effect attacks by 18%.
Level 3

Rejuvenate
Armor
Receive 30% more Healing from other players.
Level 3

Kill to Heal
Armor
Getting a kill or elimination heals you for 600 Health, unaffected by Anti-Healing.
Level 3

Chronos
Armor
Reduce the Cooldown of all your abilities by 30%.
Level 3
This has become my main Ranked Build.
This build is still for a point tank playstyle, but it focuses on punishing any very high-health champions. You don't need this one going against Double Tank, but it's usually a good bet. If the Double Tank is a Barik and Torvald, maybe go with Dark Influence. The playstyle is pretty much the same as Dark Influence, but it's unintentionally a fantastic Ult Spam Build.
For Items, get Haven 1, Morale Boost 1, Rejuv 1. After you have those three, focus on Haven. Then split priority between Rejuv and Morale Boost. Your last pick is situational. Kill to Heal or Nimble are my recommendations.
Note: I just did a match against an Azaan, Ash, Furia, Buck, and Omen. I had a double support Rei and Grohk and we were UNSTOPPABLE. I got Moral Boost right away and I was ulting 2-3 times per round. And I just never died.

Show Of Force
Rift Slash Cripples enemies it hits for 2s, and the resulting Abyssal Rift lasts 1s longer.
Level 1

Center Of Combat
Royal Presence
Increase the max health damage of Royal Presence's Poison aura by 10%.
Level 5

Abyssal Authority
Increase the radius of Royal Presence's Poison aura by 100%.
Level 5

All Will Kneel
Rift Slash
Increase the Abyssal Rift's pulse area of effect by 12%.
Level 1

The Little Things
Reduce the Cooldown of Rift Slash by 0.4s, up to a max reduction of 5s, when hitting enemies with Realm Breaker.
Level 2

Forever Changed
Increase your maximum Health by 300.
Level 2

Haven
Armor
Reduce the Damage you take from Direct and Area of Effect attacks by 18%.
Level 3

Morale Boost
Armor
Increase your Ultimate charge rate by 30%.
Level 3

Rejuvenate
Armor
Receive 30% more Healing from other players.
Level 3

Kill to Heal
Armor
Getting a kill or elimination heals you for 600 Health, unaffected by Anti-Healing.
Level 3
If you like a good shield dancing playstyle, this build is for you. With this one your main damage output is your inhand weapon. I recommend this if you're going against a Terminus or any close-combat focused champion.
Ideally the enemy would dance with you and be affected by the 10% bonus damage, but in my experience that usually isn't the case. The 2 second cooldown reduction is still solid, though! Show of Force is always a solid go-to if you don't wanna risk it.
Rejuv shouldn't be as high of a priority as movement speed is very important for this build. But you still want to give it some attention. First picks should be Nimble 3, then build Rejuv and Haven before getting Chronos.

Abyssal Breach
Enemies that pass through Abyssal Fortress are Revealed and take 10% additional weapon damage from you and your allies for 3s. Reduce Abyssal Fortress' Cooldown by 2s.
Level 3

Devastating Blows
Weapon
Increase the trigger distance of Realm Breaker's bonus damage by 15 units.
Level 3

The Little Things
Reduce the Cooldown of Rift Slash by 0.4s, up to a max reduction of 5s, when hitting enemies with Realm Breaker.
Level 2

Frontline Commander
Increase allies and decrease enemies' Movement Speed by 28% for 4 seconds when passing through Abyssal Fortress.
Level 4

A Swift End
Increase your fire rate by 20% for 3s when passing through Abyssal Fortress.
Level 5

Forever Changed
Increase your maximum Health by 150.
Level 1

Nimble
Armor
Gain 21% Movement Speed.
Level 3

Haven
Armor
Reduce the Damage you take from Direct and Area of Effect attacks by 18%.
Level 3

Rejuvenate
Armor
Receive 30% more Healing from other players.
Level 3

Chronos
Armor
Reduce the Cooldown of all your abilities by 30%.
Level 3
This is my ATTEMPT at an Off-Tank Nyx guide. I'm not crazy good, so take my advice with a grain of salt.
In my experience, Nyx is best as more of a Hybrid Tank than a pure Off-Tank. And remember: You're still Nyx. She's best as a support, not a solo. Team up with your Damages and Flanks to support the sidelines, and help out the Point Tank on the point whenever they need it. Retreat early, remember: little mobility.
This build specifically has the strategy of keeping people where you want them. Rift Slash to trap them, then right as they're about to leave the Rift, Chain Pull them back into it to finish them off. You technically could achieve a similar effect with Subjugation, and then replace Face to Face with something different, possibly Brutal Pursuer or Devastating Blows. However, I think the cripple and duration increase Show of Force is more useful.
Forever Changed could be replaced with Devastating Blows in the main build.
Your item priority is Nimble, Kill to Heal & Rejuv, and then Haven. If you're focusing squishies you shouldn't need Haven right away. REJUV IS VERY IMPORTANT AS AN OFF TANK. Your support will thank you. Remember Nyx's passive ability can help you recover faster the closer you are to the point!

Show Of Force
Rift Slash Cripples enemies it hits for 2s, and the resulting Abyssal Rift lasts 1s longer.
Level 1

Unchecked Power
Rift Slash
Increase the slow strength of the rift's pulses by 25%.
Level 5

All Will Kneel
Rift Slash
Increase the Abyssal Rift's pulse area of effect by 24%.
Level 2

The Little Things
Reduce the Cooldown of Rift Slash by 0.8s, up to a max reduction of 5s, when hitting enemies with Realm Breaker.
Level 4

Face To Face
Royal Presence
Increase the pull strength of Royal Presence's chain grab by 60%.
Level 3

Forever Changed
Increase your maximum Health by 150.
Level 1

Nimble
Armor
Gain 21% Movement Speed.
Level 3

Kill to Heal
Armor
Getting a kill or elimination heals you for 600 Health, unaffected by Anti-Healing.
Level 3

Rejuvenate
Armor
Receive 30% more Healing from other players.
Level 3

Haven
Armor
Reduce the Damage you take from Direct and Area of Effect attacks by 18%.
Level 3
Honestly Nyx is pretty strong against most characters. Terminus is her biggest counter by far, but any Low-Health tank takes down her viability.
Koga is only really annoying because of his cleanse with his dash.
Other than that percent-based-damage, distanced enemies, and executes are your biggest worries.
Even so, snipers aren't too strong since you can just shove your shield up.
Except Skye. You can use your Royal Presence like a bootleg Illuminate on her.
Anyone with a decent amount of health or a reliance on mobility is good for Nyx. The only issue is getting close enough to get them with your Rift Slash before they run away.
Nyx can be either an off tank or a point tank. Inara and Azaan work great with her just because of the massive amount of sustain they'll have together. Raum is nice because he'll heal her while she's on point, and her Rejuv will make it more beneficial.
For supports, anyone that can heal gigantic amounts quickly is great. Those who can increase her movement speed are also fantastic.
With Damages and Flanks, Nyx will be helping them get kills by slowing them down and dealing supporting damage. So really anyone is able to benefit from that.