Brief explanation of my choices
Firstly, the pick of Ring of Protection gives you more survivability in the early game. The tangos and healing salve should keep you alive as well. Your iron branches not only give you more of you most important stats however also will be turned into a wand (which can be used for more mana and hp in a fight). The ring of Basillius is then picked up for extra pushing power as well as more agility when you turn it into a Ring of Aquila. The Ring Of Aquila will only help you in the early to mid game as the agility factor of it will not be needed due to your Essence Shift. All the items i have picked will give more attack speed to Slark, except from Abyssal Blade. That's because Essence Shift is a stackable ability, which gives more agility the more you hit. This gives more damage and armour as you go.
I got the Pounce at the start as it reduces the cooldown and increases damage done. It will be useful for constant ganks/escapes. Shadow Dance should always be upgraded as soon as possible. You can go to dangerous situations and heal up quickly. This leaves less risks in ganks and being aggressive in lanes. As weird as it seems, Dark Pact is one of the most useful abilities. In the End Game, lots of people may have negative buffs and removing them gives you an advantage, and in the early game the damage done is amazing which will help you to score kills.