This guide is just what i use when playing Slark.
It cuts out a lot of the Valve default items, because i don't think as a Slark you should be purchasing health regen items... That's what your ulti's for.

The main items are Power Treads (you can change this if you like) with Sange & Yasha + Shadow Blade.
Shadow blade is good for escapes + activates your ulti's passive + has nice bonus damage. And Sange & Yasha is just too good an item for Slark to pass up.

This build also prioritises pounce over dark pact