Night Stalker, also known as Balanar, is a strength melee hero infamous for reigning over Dota 2 nights. When picked in a game, this hero makes everyone care about the day and night cycle and he is unique in that. While he seems to be below a hero during the daytime (especially the first day), the wolf's howl releases the beast and he becomes the most feared one on the map.
After suffering a series of nerfs, at last, Night Stalker experienced two major changes in 7.20 patch which are considered great buffs, a rework to Crippling Fear and a replaced ultimate ability Dark Ascension with its talent option.
To have a quick preview, he has a mini-stun followed by a slow ( Void), a mobile AoE silence ( Crippling Fear), a strong passive ability giving movement and attack speed bonus at night which is also the hero's signature ability ( Hunter in the Night) and his Ultimate ability which turns the day into night and gives him unobstructed movement/vision with a decent attack damage ( Dark Ascension).
Now let's check his abilities with some key points.
Your only nuke with a relatively good cast range and good damage growth per level. This is your ability to start the game with. Level 1 Void doesn't often worth using in the first day because it would deal about 34 damage points (considering halved to 45 and reduced by 25%) so as you've noticed, this ability usually needs to be maxed out first to be most effective.
During daytime, since it has halved damage with no mini-stun, don't use it carelessly unless you see the opportunity to close your gap and trade attacks with the enemy hero because there are really few heroes who can successfully trade attacks against NS and most of your opponents would avoid it. If you are in a ganking situation, it's important to be patient with your opponents and not to use Void too early or they will teleport away easily. Also keep in mind that the mini-stun interrupts channeling spells like Death Ward, Fiend's Grip or etc. and because you're on the frontline, you can easily reach those heroes causing the trouble.
A unique type of silence in the game! It's an Area of Effect silence so it cannot be dispelled and the only solution would be Black King Bar. Moreover, there are few AoE continuous silence sources like Static Storm or Smoke Screen and the thing is that enemies can walk out of them but Crippling Fear aura is set on your hero thus it has mobility, and you keep enemies silenced for the full duration by just chasing them, which is Night Stalker's favorite job and also remember if maxed out and used at night, it is the longest silence in the game (8seconds)! One level of Crippling Fear before level 10 is usually enough, then it would be postponed until level 13 is reached (surely there are exceptional situations). This ability helps Night Stalker to be fit for the frontline in teamfights and it's suggested to opt for the tankier item builds to survive if you have to maneuver between enemies with this ability for your team (for example, when your team lacks silence).
A passive ability to dominate the map at nighttime! This ability is usually leveled up after and along with Void and even has priority in some situations. It's crucial to have at least one point in this ability before the first night at minute 05:00 (especially for a #4 Night Stalker who is expected to be underleveled). Most of enemy heroes can't resist against your attack speed and the movement speed bonus helps you to rotate and gank faster than anyone else.
The dark knight rises! Turn the day (if it's daytime) into night and benefit from your overpowered nighttime abilities whenever you want. Need more? Ok, here is 50/100/150 bonus damage to even tear apart enemy tanky heroes. Now you should be satisfied with it but good news is there's more. Unobstructed movement and vision to make sure nothing can stop you and there would be no juke plays by enemies.
The game timer won't be stopped when you activate this ability, so try to use it as soon as possible and keep it off cooldown, I mean you should manage to put yourself in a ganking/rewarding situation whenever you notice Dark Ascension is at hand; for example, you see that Dark Ascension is on cooldown for about 30 seconds so you inform your allies to push a lane and at the time they are about to take a tower, you would have your ultimate ready (so you can use it to scare your enemy and bag the objective safely or save it to surprise your enemies in daytime if they try to defend.), and at night, try to use Dark Ascension wisely, for example if possible, keep it ready to have the kill potential for an enemy carry or tank in the right moment, or to just prevent them from taking part in a fight because they should really care about your extra bonuses from your ultimate ability.
Let me skip the starting items which are mostly the same for many heroes, but you can use this advice: Buy a Stout Shield if you are #3 NS or if you would be harassed by a long range enemy who you can't trade attacks with (like Sniper), buy a Quelling Blade if you are #3 or #2 NS against enemy with high attack damage (like Tiny), prioritize buying Enchanted Mango over Clarity, buying an orb of venom while facing a speedy opponent allows you to keep attacking slowed enemy without using Void saving it to cancel tp.
Smoke of Deceit : If not you, then who?
Bottle: No matter which role you are playing, this item is incredible for NS allowing you to bottle useful runes for ganking although #4 NS is better to opt for this item only if being in a snowball situation. You can always fill it because the enemy supports usually don't dare challenge you for a rune.
Boots: Phase Boots give you some attack speed, more tankiness and the ability to run through units with extra movement speed and are best to be bought by #3 or #4 NS while a midlaner NS would benefit more from Power Treads 30 attack damage as a semi-carry. Early Boots of Travel can be bought by a midlaner NS to contribute in more ganks.
Urn of Shadows: A good item for any ganker with satisfying mana regen. Its upgrade, Spirit Vessel, is a strong counter against heroes that rely on healing (like Huskar)
Mordiggian: Best damage per value item for a strength carry, also making you tankier.
Drum of Endurance: An all-round team support item for an offlaner or a utility support. Boosts stats and gives movement speed and mana regen with an active for chasing or running away.
BKB: best choice against a team with a lot of disables
Skull Basher: due to your high attack speed at night, usually procs and it's like a slap in enemy carry's face if bought early. The upgrade, abyssal, makes you tankier and grants a bkb piercing stun.
Aghanim's Scepter: Gives full range unobstructed vision at night which is great for a team to plan for a gank or to clear the map combined with a gem. If you need it in a game, queue it as soon as possible.
Crimson Guard,Pipe of Insight,Vladmir's Offering,Assault Cuirass: Situational supporting items buy Crimson for early pushes or when the enemy team are more reliant on attack speed without high attack damage, buy Pipe against magical nukers (like Zeus), opt for Vlad's when you have many attackers on your team , AC makes your allies tankier and boosts your team's pushing power.
Blink Dagger: Helps you to blink into the enemy backlines but more importantly, you can activate Crippling Fear and then blink into enemy team, initiating a fight by silencing multiple enemies.
Shadow blade: Similar to dagger with bonus attack damage and attack speed but can be countered with true sight.
Heaven's Halberd: A handy item disarming enemy attackers and also boosting your survivability and damage output.
Blade mail: activate it while closing in on a squishy target with high damage output (like Sniper or Drow Ranger)