For me when i build Pantheon i build him as Armor Pen & Attack Damage , i start off with a Doran's Blade. and start farming from their once i get about 800+ gold i go back to base and buy one more Doran's Blade and Boots. now i'm ready to gank & harass. When playing Pantheon you want to harass with spear and stun to get kills. When i get 2000 gold i go back to base and get Boots of mobility to chase down my enemy. As for Brutalizer that's my Armor pen ingame.Then back to hunt for gold till i get about 1875 gold to get my Yomumu Ghostblade for movement speed and attack speed. and one Vampiric scepter for my life steal.I save enough gold to buy my bloodthirster.And if i'm facing and tank i would get a Last Whisper if i'm not when i would go for my Phantom Dancer.And soon sell one of my Doran's Blade and get another Bloodthirster.And start to build my second Phantom Dancer.Lastly my end game item would be Warmongs because Pantheon is very squishy late game so you would get some health to put on that.