Black Cleaver does not work with HSS. It also does not work with Spear Shot. It procs off of attacks, and neither of those abilities count as attacks. An easy way to test it is to grab the red lizard buff and see if your skills slow people. Neither of Pantheon's do.

Also, you shouldn't be building Black Cleaver and Last Whisper, you should commit to one or the other. Black Cleaver gives less AD than the other BF. Sword variants and it's only draw is stacking flat ArP. Last Whisper will be less useful the more flat ArP you stack because it's a percentile based reduction.

Just a few things I noticed right from the get-go.

Great build
It combines survivability and damage
+1

nice build..but are u sure black cleaver works with hss? I don't think it doeS