Zeratul can be difficult at first until you get the hang of him.

I've played with several builds & find that while many players prefer choosing "Greater Cleave" for their initial talent, Seasoned Marksman is the far superior way to go as late game scaling or games vs AI or Hero League you will often find yourself with 30 - 50 stacks by level 20+

The best part about Seasoned Marksman is you can sit stealth nearby & let your stacks build up if you find yourself low on health or 1 v 2 in your particular lane.

I would recommend saving Blink as your escape rather than as a means to engage opponents, unless you're chasing down an opponent with low health & can guarantee that you'll land the kill.

Block is relatively useless because Zeratul is so squishy that if you're taking massive amounts of hero damage you're likely going to be dead anyway, & you should be carefully (albeit aggressively) choosing your targets.

If you find yourself alone in a lane of minions or able to cleave a wave & pick them off from safety, definitely do so, as it will rapidly build stacks. As you start piling up ganks on Heroes, you'll find that you get considerably more stacks.

The reason for taking Seasoned Marksman over other talents at level 1, is because when you need that extra damage to secure a gank or push a tower or keep, the overall damage you gain from it is arguably the difference between securing a kill or an objective or forcing opponents to run & scatter during team fights.

In the late game, Seasoned Marksman will make you downright lethal.

This talent is obvious & self explanatory. It initially gives you a 5% boost to all of your abilities, & stacks to a maximum of 15% at 5 hero kills (or assists), resetting upon death.

When combined with Seasoned Marksman , your overall sustained damage from minions, plus the burst you gain at 5 stacks of Gathering Power makes this the best talent choice for this tier.

Some players would argue that Focused Attack would a better choice with Seasoned Marksman & I would argue otherwise. Zeratul is a stealthed hero who has insane damage & is VERY squishy. If you're out of stealth long enough to be landing enough basic attacks to proc this (or only breaking stealth every time it's reset) you're still not going to do as much burst (or sustained damage) as you will receive from getting 5 stacks of Gathering Power, provided you learn Zeratul well enough to stay alive once you have them.

All of the other talents in this tier really don't lend well to the hit & run strategy.
Gathering Power is by far your best bet.

I was on the fence about my choice here, as I rarely take First Aid. While having the 35% heal is nice, overall your build is focusing on bursting down minions & heroes, before retreating into stealth.

Void Slash gives you an additional 30% boost to damage if used while stealthed. Combine that with 5 stacks of Gathering Power & that's a 45% damage boost to that cleave, which can really turn the tide of a team fight.

Shadow Spike can definitely be useful too, as the range can often catch a fleeing hero or healer & the ability to pop it off from stealth is nice (especially with Double Bombs later on). The problem with Singularity Spike & the reason I opt NOT to take Shadow Spike is, that it can be very difficult to land on your intended target when you're dealing with lane minions or a team fight, because Singularity Spike hits the first target within it's straight line of fire. End result you will more often than not end up missing your intended target, or hitting the wrong target and the additional range or not breaking stealth becomes negligible at that point if you're not able to land the special.

First Aid can seem like a very appealing talent, & should definitely be considered if you're finding yourself against very tanky comps, like Muradin or Sonya , or if your team is without a healer, or you find that you're having a lot of difficulty staying alive.

Overall Void Slash is my talent of choice, because that additional 30% boost from stealth can literally rack you up 5 kills on a team fight, if you've got full Gathering Power stacks.

Void Prison has so many uses, but can be tricky to use in a team fight so that you don't accidentally remove a teammate from the fray either. Still I would recommend it over Shadow Assault , as Shadow Assault requires you to remain out of stealth the entire time it's being channeled, and often leaves you very dead.... Zeratul is squishy... The longer you are out of stealth, the more liable you are to be very dead.

Void Prison however, gives you the opportunity to remove a healer from a team fight, or single out a fleeing hero, & block their escape route to secure the kill.

At level 20 you can optionally take the additional Void Prison Talent which allows your teammates to move through the prison while it's still holding your enemies which can be incredibly powerful.

With Seasoned Marksman at level 1, Gathering Power at level 4, Void Cleave at level 7, this is just a no brainer to me....

It really really gives Zeratul the ability to burst down nearly anyone in a team fight. Even the more dangerous tanks like Diablo, Sonya, & Muradin will often run from you if you can catch them 1v1 when their heroics are on cooldown.

It also makes support heroes like Tassadar who you would normally run from much less dangerous & far easier to engage 1v1 & far more likely that you'll be able to secure the kill.

Giant Killer is my pick hands down.

Burning Rage is by far the worst talent in this tree, as dealing damage to things around you constantly pops you from stealth. It's self-defeating to the point of having a stealthed assassin, having a talent that constantly breaks you from stealth every time it deals damage to something.

Wormhole can be useful if you find yourself needing the extra escape, so don't rule it out. It's definitely the more popular choice for most players. I personally find that the additional blink often gets me killed more than it helps save my hide.

Assassin's Blade can be equally useful for mobility and that initial boost from breaking stealth, but overall Giant Killer is better as it does attack damage 1.5% vs their max health & when you combine that with Seasoned Marksman ..... That gets pretty sick.

Being able to fire Singularity Spike twice can often deal enough damage to even a tank or healer that causes them to disengage from a team fight or an all out team push.

Berserk can seem appealing & definitely offers a fair amount of burst, plus the added mobility, but ultimately you will gain more damage overall from two singularity spikes landing on the same target with this build.

Stoneskin = No. If you're taking enough damage to need this, you're playing Zeratul wrong.

Rending Cleave = It can be nice having that additional 50% over 5 seconds, but ultimately the only way you'll gain the full benefit of this is provided you land it on heroes from stealth for the 30% Void Cleave bonus with full stacks of Gathering Power . Otherwise, you're better off just taking Double Bombs as it's a set amount of damage at 5 stacks & the range on it will at least let you hit a fleeing target once or twice.

Double Bombs is almost always my pick for Tier 6.

I would recommend this over Protective Prison unless you're playing in Hero League or Hero Team League, as it will cause all of your attacks to slow their targets (including enemy heroes).

When you factor in Giant Killer , Gathering Power , Seasoned Marksman , and your other talents it really makes Zeratul downright lethal during the late game & gives you the added edge to be very aggressive.

Rewind can seem appealing too, because of the way it can be abused with Double Bombs , but it really does you no good with this build once you're out of stealth.
Namely because you lose out on that 30% bonus to Void Cleave which makes the damage from Rending Cleave (if you took that instead) negligible & to my knowledge the damage over time effect doesn't stack.