Not Updated For Current Season
I begin with meki pendant and 2 hp potions.With full seals/glyphs mana reg per level you should have about 20 or something mana regen at level 4 or 5.Which is huge.
Early Game : Your First item is Manamune.You should back when you have level 6 since you have a free teleport-Grand Skyfall.Your goal is to harass with spears and last hitting creeps with all your spells when possible.Don't be afraid to waste your mana for creeping.
Mid Game:Get your Black Cleaver and begin non stop ganking,stealing their buffs,pushing towers.Be aware though you don't have much hp until getting your sunfire.So when possible get your Elixir of Fortitude and hp potions also don't forget wards.
Late Game: Now you should have Ghostblade and started building Sunfire.At this time in the game you should be very careful when attacking/initiating a fight also don't do it very often your the or main carry your supposed to stay behind the tanks and just anticipate a good opportunity to come into the fight.Your last item should be Guardian Angel or BloodThirster or maybe what ever you choose its up to you.
I have about 150 games with Pantheon have tried different builds though this one seems to be the most cost efficient to me.