I just tried out this build on our favorite Shadow Assassin (Kayn) and I really liked it.

Let me know your thoughts and and tell me how it worked out for you if you give it a shot.

https://imgur.com/t4JaW4s

I realize trinity force is a huge order to pick up as your first item but the spike was huge once I got it. You could go duskblade first if you wanted for a quicker (and probably stronger) first item spike.

Also ignore that ruby crystal, I sold it as I was just crafting the build as the game went on. I ended up going 21/9/10 and was often killing people before I even saw the opportunity to ult.

I didn't pick up the warrior enchant because I knew I'd hit 40% cdr without it as I initially planned on building Trinity into black cleaver but decided I didn't want the double phage passive while also needing a quicker burst item since I was playing kayn instead of rhaast.

I just picked up echo last for a bit more mixed damage and a bit more burst. Seemed to work fine... not sure if the 60ad from warrior would have been better though.