Wanting to make a dps nail Assassin, clueless o.o

So I saw a video of a solo Assassin called Seknz on YouTube, and I wanted to make an Assassin xD the thing is I'm unsure of his build o.o he had 11k hp and a lot of int. He had a lot of buffs on all, I saw was darkness and he kept on spamming shadow walk? (I'll try to post a link when I get on a computer)
But I wanted to make an Assassin I'm just unsure if I should max agi crit or have a mix, and I'm also not sure what equipment to use. Oh btw what does shadow walk do exactly

Heres the link