I played DotA since the first Frozen Throne version, and then I played Dota 2 for quite some time. However, now I decided to play this simply because I have less time and matches here are much faster. Also, I Starcraft And Warcraft 3 are the games of my childhood, so there is ton of nostalgia.

Anyway, Zeratul is pretty much what Riki (Stealth Assassin) is in dota. You are supposed to roam around the map and help your teammates, as well as take down low targets by yourself. Always be patient and careful, and pick your targets carefully and kill them fast. This is a really easy hero to play, as long as you have the right playstyle. You should never attack minions, that is not your role. You should never stay in a lane, you should always roam in between, as well as help in teamfights by hitting their squishy heroes. I found out that this build is the best for that. Also, never hesitate to use Void Prison. It's just so strong, and it can help in so many situations. Even though Zeratul is very squishy, you should still have very, very few deaths in matches.

T1: This is super strong talent. Reduced blink duration helps a ton. I know 1.5s seems like not a lot, but it makes all the difference.
T2: It's either this or Vorpal Blade. I find this to be more useful though for teamfights, but also when someone is alone they dodge it harder.
T3: You really need this. It helps you stick around much longer. Very important talent for this hero.
T4: If you know how to use it, along with Tassadar's wall, this is the best ultimate in game.
T5: 25% damage boost is obviously a lot. But what's also important is speed boost, it lets you roam around the map much faster. You need this.
T6: This really depends how good you are mechanically. If you can make the most of this talent then good for you. But if not, Stoneskin is also a good choice. Rending cleave is acceptable choice as well.
T7: Makes OP ultimate even more OP. With this you can win any team fight very easily.