On Night Stalker, it is important to have a ton of burst damage so that it is easy to towerdive and pick off heroes during the first night. Taking Crippling Fear at level 2 is recommended and I think that it's important, but it can be skipped for an extra level in Hunter in the Night.
With Darkness, there are generally two accepted choices. Either don't level the skill at all until after the first night, prioritizing your Hunter in the Night, or take one point right at the start of the first night and use it right away. Remember that using Darkness during the night time will freeze the game clock and thereby prolong the night time. All of Night Stalker's skills are useful and important, so avoid taking stats until level 15.
Night Stalker has a few options with regards to laning. He can be successful in the middle lane, especially against passive heroes like Nature's Prophet. He is even suspenseful against more active heroes like Puck and Windrunner. However, he doesn't do nearly as well against heroes with lots of harassment like Queen of Pain, Magnus, Batrider, as well as most dual mid lanes. That being said, it's still possible to beat each of these lineups, it will just take very careful execution.
Another place for Night Stalker to lane is in the safe lane with a support hero. While this often will delay levels (as you are sharing some experience with your laning partner), it may result in some early kills if you can combo well with your support hero. It usually also guarantees that Night Stalker will be secured safe farm in the early stages of the game, making sure that he gets a bottle and some boots in a timely fashion.
First night is one of the most important times for a Night Stalker During this time, it's important to do a few things. If your team has decent map control and good ward vision, it's important to try and win your other lanes. If one of your sidelanes is struggling, it can be a good idea to team up with your supports or with whoever is in that lane and try and secure a kill, even towerdiving when necessary. If you can manage a good gank between the 6 and 8 minute mark, often support heroes or mid heroes won't be carrying teleport scrolls yet. However, if there is TP support coming when you are diving a tower for a kill, it comes down to an important judgment call. Can you get the kill and run away before the support arrives? Do you have enough HP to fight the support that is coming? Is it better to run away and not die under the enemy tower?
Each of these questions needs to be analysed and answered often in a splitsecond, so it's important to keep a level head and make the right decision. Choosing when to go for the dive and when to retreat often makes the difference between a successful first night and an unsuccessful one.
Another potential target for Night Stalker in the early stages of the game is junglers. Heroes like Lycanthrope, Enchantress, Chen, Enigma, and Nature's Prophet are often alone and far away from TP support while farming the jungle. Sometimes they're even quite low health. Keeping an eye on the game to determine whether jungling heroes are farming or ganking can pay big dividends if you can orchestrate a gank against them.
Last but not least, carry a TP. Night Stalker is not only good at ganking, but also good at counterganking. If you have a hero in trouble, even just one teleport from a Night Stalker can turn the gank around for your team.
Towards the midgame, continue ganking when possible (especially trying to pick off warding support heroes). This isn't always possible, however, because teams often start to group up as five and look for bigger team fights.
When it comes to team fights Night Stalker doesn't have as many tools as other heroes. However, that doesn't mean he can't be useful. A well placed Crippling Fear will prevent enemy heroes from casting abilities, potentially turning the tide of the fight. Also Voiding a hero channelling a spell will also help your team. Lastly, if there's no big target that requires a silence, consider using Crippling Fear on the enemy carry. While this does silence them and can often be important, it will also cause them to miss a portion of their attacks, limiting the effective damage they can deal.
As for pushing with your team and taking towers, Night Stalker can be a huge asset. If you've managed to complete an Aghanim's Scepter, your team can push with confidence, gaining unobstructed vision around Night Stalker. This can be an early warning to spot out ganks or even find potential kills.
In the late game, use Night Stalker more as a utility hero. His silence and Void are still useful, but won't have the same effect against enemy team fighters or carries that have built Black King Bars. Even so, pick off the supports when possible, and try and take as much damage as possible for your carry so they have time to deal a lot of damage.
Night Stalker's primary role is to gank. Therefore, it is important to build items that help him accomplish this role. Items like Urn of Shadows and Bottle give burst heal which is important for when Night Stalker is diving towers. Additionally, controlling the runes can really help surprise the enemy team and will keep your manapool high.
In the early to mid game, think about picking up either a Eul's Scepter of Divinity or Aghanim's Scepter. They will both help with ganking. Euls will give you extra movespeed, mana regen, and a disable, all of which Night Stalker needs. Aghanim's Scepter will give Night Stalker unobstructed vision during the night, which really helps with chasing and ganking enemy heroes. If possible, pick up both.
Black King Bar is entirely dependent on the amount of enemy disable. If your silence is enough to prevent you from being disabled, then skip BKB. Vangaurd is also dependent on the enemy team and whether you feel like you need some extra health.
Never get Mask of Madness or Battle Fury. Night Stalker can build Battle Fury and Phase Boots and win games. There's no doubt about that. However, the reason builds like this are called into question is because that's not the role to which he's best suited. Just take a look at his skills:
Void is a great nuke, one of the best in the game. It's a good slow, and does above average damage for medium manacost and low cooldown. This is by far Night Stalker's best skill because it lets him burst down heroes. At level 7, Night Stalker's base attack is 79-83 dmg. If you have Void lvl 1, you will do 90dmg nuke and maybe 3 hits for a total of 324dmg. Void level 4 does 335 damage alone. Maxing Void is clearly the correct decision to make, even if you're sacrificing a little bit of movespeed and attack speed by not leveling Hunter in the Night.
As for his role, he is a great ganker. With extra nighttime movespeed, a high-damage nuke and slow, coupled with silence, he can tower dive quite easily to get kills with minimal support from the other laning heroes. Therefore, he should build items that suit this role.
One of Night Stalker's main weaknesses is his lack of mana. He has a small mana pool with lackluster intelligence gain. Therefore, he should get something to help him regain mana and something to help him regenerate health after diving towers and killing heroes.
If he's doing a good job ganking, he'll be spending considerable amounts of time in the river, going from lane to lane. At this stage it makes sense to grab a Bottle. Bottle gives 210 mana and 405 health over 9 seconds. Urn of Shadows is also useful because you can use it offensively and defensively. It can either be used as a 400 hp heal (useful for both you and any allies in other lanes) or as a 150dmg over time, something helpful for finishing off fleeing heroes.
Battle Fury on the other hand, gives +6hp/s regen and +150% mana regen. This means that at level 7, it will take Night Stalker almost 2.5 minutes to regen from 0 to full hp. With an urn and bottle, however, you are not only saving almost 3k gold, you are also going to heal to full in about 18 seconds (With two urn charges and a bottle). If the mana regen seems like it would be beneficial, that's also false. A level 7 NS has a base mana regen rate of 1.0mp/s. With Battle Fury, he has 2.6mp/s. With Urn, he has 1.6mp/s. When you couple the Urn with a Bottle, Night Stalker has a way faster regen. Therefore, there are only two benefits to building a Battle Fury - cleave and +65 dmg.
The main problem with a Battle Fury is aquiring it. Farming a Battle Fury is a problem for Night Stalker. If you last hit every creep in lane and don't buy anything else, it will take over 13.5 minutes to "rush" Battle Fury. At that point, night has ended and it's a day cycle again. However, if you do the same thing with bottle and urn, you can finish both and boots in just over 6 minutes. Realistically, you won't last hit every creep. Therefore, you're more likely to get your Battle Fury around 20 minutes and Urn of Shadows, Bottle, and Boots of Speed around 8.5 minutes.
Lastly, if you gank during first night with Perseverance (more expensive than the cost of Bottle and Urn) you either can't be aggressive to get kills, or you have to go back to base to heal.
Overall, Battle Fury is a weak item choice on Night Stalker for a number of reasons. If you'd like the justification for items like Aghanim's Scepter and Eul's Scepter of Divinity I'll be happy to provide those reasons as well.
To start off, let's identify Night Stalkers's biggest strengths and weaknesses. His main strength as a hero is his ability to lock down almost anyone in a 1v1 situation at night. His weakness kicks in when the enemies outnumber him, effectively disallowing him to go for his pursuit.
Eul's Scepter of Divinity enables you to enter 1v2 situations, knowing that you can cast Void on the target, cast Crippling Fear on the target, and use Cyclone on the remaining one. Even in a 1v3 situation, you can cast Void on the target, silence a secondary opponent, and cyclone a tertiary target. Remember also that you will often have the backup of at least one allied hero. With Eul's Secpter of Divinity you can exploit poor positioning of the enemy team.
Secondly, Night Stalker needs mana regen and mana pool to effectively be able to consecutively chase across the map without having to return to base in order to regenerate mana and health. With Urn of Shadows for the both the self-heal and enemy DoT as well as Eul's Scepter of Divinity, Night Stalker can remedy many of his weaknesses.
The Eul's Scepter of Divinity is not only powerful when ganking, it also provides incredible potential in team fights as well. For example, a hero like Tidehunter wants to control the outcome of a teamfight by casting an effective Ravage. However, an item like Eul's Scepter of Divinity can immobilize Tidehunter for 2.5 seconds. Couple that with silence, and Tidehunter is out of the fight for 10.5 seconds. Furthermore, heroes like Sand King and Witch Doctor rely on channeling ultimates ( Epicenter and Death Ward respectively) to be effective in teamfights. A Eul's scepter can quickly break that channel.
Lastly, you can cast Cyclone on yourself. Cycloning yourself is not specifically awesome to Night Stalker in contrast to casting on other heroes, but Night Stalker is a hero that needs to be locked down with disables during ganks. If you're able to mid-air dodge Magic Missile, Storm Hammer, or Earth Spike (among others), you can pursue afterwards in the teamfight or gank situation. Unlike other heroes, Night Stalker can actually catch up after landing from Cyclone. Other heroes can only use the item defensively on themslves. Balanar can use it hybrid defensively/offensively.
Nov 25/2011 - Added Guide
Nov 27/2011 - Added Eul's Scepter Chapter
Feb 25/2013 - Added 2 chapters, adjusted items