Just general tips on his build/items/strategy. When is the best time to use his ult? And what exactly should I be doing during the daytime? I tried him out in a pub game last night with CB, shake, Crow and pheenix and did pretty well. Once the first night hit, I got first blood and started roaming the map for ganks. The silence + slow is amazing against magic users. Got quite a few kills, but then they started focusing on me late game. The rest of the team cleaned up though.
That's generally how NS games go. He's super effective once that first night hits, and continues to be when the second night hits too. Once the third night comes around, he starts to drop off a bit because heroes start to get more hp/items. But if you get a lot of kills in those first two nights, you should have enough of your own items to still stay effective somewhat in the late game. If you get behind though, you become quite worthless except for your silence.
As far as skills go:
Always max your nuke (Q) first obviously, put 1 point into silence, and then the rest into your passive (so this gets maxed after your nuke). When you hit 6 though, it sort of depends on how the game is going for you. I generally hold off on leveling Darkness until 7 or even 8 sometimes. The extra points in your passive are really good early on because it gives you that extra movement/attack speed to chase guys down in that first nighttime. And if you are getting good ganks off, you should get those extra levels pretty quickly. As soon as you level your Darkness, pop it right away. Think of it as just extending your first night a bit longer. Generally people think they should save it for when daytime hits, but you want to use it whenever it's up during a real nighttime. It will maximize the amount of time you actually have at night this way. When daytime does hit, that's when you become situational with it. So if your team is about to engage or roam for a gank, you want to pop it only when necessary because the enemy team will know when you do. It will blow your cover so be careful with it.
Another good tip is that your nuke (Void) is also a mini stun. So if you are ganking someone and they are already running for the side shop or whatever, hold off on using it if you know they are planning to TP. The cooldown is only 8 seconds though which is really good on that spell.
Item builds:
I will always start out with two gaunts of strength, tangos, a healing salve, and a couple sticks. You will want to build those 2 gaunts into an Urn almost right away (goal is to have it by 6 min for when night pops and you start ganking to collect as many charges as you can). Those Urn charges are so important early on because they can keep your health up so you don't have to return to the base ever (thus maximizing your time ganking in the first night), and they can also finish off enemy heroes as well if you can't dive past towers while they are running away at low health. Magic Wand is also a good item to build after the Urn if you can (although if you can't finish that and boots by 6 minutes just get boots before it since movement speed is so important early on). Urn + Magic Wand will give you that extra HP/mana to help keep you out and about. I should also mention that Bottle is a very good item as well. If you want to, you can buy one early on, but I really like having the early Magic Wand and charges depending on the heroes I'm going up against if they cast lots of spells to charge me up. Bottle is good for obvious reasons, and Nightstalker is obviously super good at running down runes since he will be the fastest hero on the map early on. Phase boots come next. Gives good early damage and enables you to run through creeps/units to get to your targets faster.
So once your early items are done (Phase, Urn, bottle/Magic Wand) you can go a couple different ways. If you feel like you aren't tanky enough, go ahead and make a Vanguard. I should also mention that the stout shield is a fairly good pickup on its own because of all the diving you do and all the creep aggro you will inevitably attract. Or, if you wanna go for a more damage focused build, go ahead and build an Armlet (get good at toggling and you will be nearly impossible to kill when you are running around at night). If I go Armlet though, I like to get a Helm of the Dominator right after in order to keep my HP up. You want to have these items by the second night or somewhat early on into it.
Once the second night ends, this is when your Void nuke really starts to drop off, and it becomes much harder to find kills because generally the other team is grouping up by now. When that happens, your effectiveness really drops off. So what I generally do to combat that is by making a Basher. This enables me to still be effective and focus certain heroes (usually the carry so I can lock them down and annoy them with the bash proc), and then I can throw up my silence on whichever caster I feel is the most detrimental to my team. Hopefully, if you've done your job well in the early and mid game, you've created a lot of space for your carry to farm and now they have some good core items in order to successfully start putting out some solid damage in team fights.
BKB is also a really good item to have. Generally good to get it in time for the mid game, and still helps a lot after that as well.
Solid items for late game: Abyssal (built with the Basher that you should make before obviously), Aghanim's Scepter (this is sort of a support item. It gives you a shit ton of vision at night including high ground vision even if you are on the low ground, and it also makes you a bit tankier as well), Satanic, Assault Cuirass, and Heart are all good pickups. I generally only do Satanic if I go with that Armlet/Helm of the Dom build.
In the end, NS is pretty flexible with item builds as long as you get those early items quickly. It really depends on how good of a game you are having and when the enemy team starts grouping up together. It really really sucks if a team starts 5 manning by 10 minutes when you play him. You generally just hope that your carries are getting really good free farm because of it because you pretty much get taken out of the game when enemy teams do that. Can be really frustrating.