Hello! I'm Devilswork! I'm from Brazil (sorry for bad english).
I play HotS since Alpha, but not a hardcore player, and just starting to play hero league just now.
Zeratul is my "main" assassin character. I love the style he gets from the surprise kills.

I use 3 build for him:
Skill upgrade;
Auto Attack;
Hidden Assasin.

Look below for little more details and any question just ask on the comments!

Most used build, gives most damage with cleave and have good survival abilities.
Cleave is the most strong skill on this build, but not the only one to be used.
Play Video
In this video, i've made a triple kill just by using Cleave and hitting sylvannas, Spiked Raynor so he gets on the Tyrael's death damage, and used the ninja heroic ability to finish it. As you can see, in tier 5 I got Stonesking for more surviability, and in the last tier the auto attack damage with slow.

The abilities are used to slow and AoE but the main damage on this build is the auto attack.
It's important to stack Seasoned Mark (tier 1 talent choice) at the beginning of the game, killing Minions but beeing careful with heroes.

As the name says, it has an incredible burst right before you get out of the cloak. The main skill is the Cleave. The idea of this build is to be cloacked, sneak behind, do your burst and get out with Blink. You won't always kill the enemy, but the damage will be great. After Wormhole (tier 5) you can also blink again to finish the sentence.