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.