Firstly, meteor hammer is beyond broken on nyx. You almost always want to get this item first, even before boots most of the time. Of course, you will need to last hit to achieve this, so quelling is a must. at 2.6k gold (more or less) a 8 minute meteor hammer is achievable. At that point, nyx is literally death whenever his vendetta is up.
Lets look at the stats. Nyx is squishy, has mana issues, and needs a finisher for his assassination. Usually, the options are sunstrike, dagon, eblade, manaboots, and aghanims.
Look at meteor hammer. 10 str and intel, hp regen, and mana regen. And look at the buildup, the most expensive component is 1k gold, similar to aghanims.
And its effect? From 7.07d, its channel time is 2.5s. Following which, a meteor lands, stunning for 2s and doing 690 damage in ticks. You see what im getting at? Not only does it do damage similar to a dagon 4, it gives insane stat synergy that nyx needs it also gives a 2s stun to chain with impale. Since impale stuns for 2.8s and has a travel and lift time, a quickcast impale into meteor is 99% guaranteed to land.
Before meteor hammer, nyx had 2 further issues. 1) He could rarely get 2 manaburns off on a target like invoker or storm to kill them. 2)After a won fight, he deals 10 damage per second to tower. The storm/invoker will just zip away or ghost or tornado, and nyx will fail the solo gank.
Now, if executed correctly, nyx can vendetta, manaburn into impale, instantly channel meteor and cause it to land, chain stunning for 4.8s, enough to land a second manaburn for the kill. That is crazy good for intel heroes like silencer and od etc. Also, it does 345 damage to towers, essentially the same as nyx hitting the tower 34 freaking times. Its that good.
Now, whenever u are level 7 and and have meteor, u can punish heroes that commonly dont buy strength items until mid-late. 690 magic damage when everyone is level 7 is extremely insane. This makes nyx an insanely good invoker, pa tinker, sniper and am counter for the first 20 minutes. pa is exceptionally vulnerable as she wants to go phase deso battlefury, which causes her to die to meteor every single time.
This also makes nyx an even better counter to meepo. Previously, he had 2 aoe stuns that were almost guaranteed to land, which caused meepo to have serious issues mid game. Now, nyx can farm the meepo all over the map because he would be relatively squishy early game, especially if he goes mid. Once killing him 2 times with your midlaner, he will usually rotate to jungle, allowing u to carapace the poof, vendetta the clone taking creep aggro, and do 690 damage to all meepos which usually secures the kill easily.
All in all, meteor hammer is sleeper op on nyx and is essentially a better midas and dagon combined. After the meteor hammer, nyx can usually snowball super hard and give his team so much space that items such as blink, aether and aghanims arent that far away. This playstyle is usually better than the support / nuker playstyle as it allow nyx to be what he always should have been. A burstly assassin.
When to not play meteor nyx
If u are against : void, weaver, pl , lifestealer, wk, riki etc