Hi! This is my first real solid build, for Pantheon. It mostly focuses around the first snowball item, Sword of the Occult.

In the beginning, get a Long Sword (for building into SotO later, and additional early damage output) and a Health Pot. Try to do this as quickly as you can. I suggest this because, although it's not extremely difficult to mid with Panth, it's more likely that you'll want to gank a bottom or top lane ASAP with your ally. You'll need your first rank in Heartseeker Strike in order to do the necessary amount of damage. This will also be useful if both enemy heroes in the lane try to face check you and your teammate. The potion will help you to survive if things don't go swimmingly. If successful, port back to base and pick up some early boots. This will help you get around and run away more easily if enemy heroes harrass you early (Mundo, Gangplank, Garen, etc.).

When you get to level 2, throw a single point into Aegis of Zeonia and leave it there. This enables you to perform the main combo of Pantheon--Jump in and stun, then Heartseeker for extreme direct damage. This can in some cases score you some early kills, but I'll save that for later. It also enables you to start maxing HsS and his Spearchuck ability, which you'll need for early farming and zone harassment. Just keep last-hitting and using HsS, just be a bit conservative with your usage, as it consumes a relatively large amount of mana to cast, and the cooldown is pretty big for a normal skill. BUT! If you do run out of mana, that's what Clarity is for. Don't be afraid to use it, as it's cooldown is pretty short, but try to be near your ally (if they use mana) so that you help them as well. Just use HsS to farm and Spearchuck to harrass, both lightly, and you should be fine.

Once you get enough money, port back and pick up either Berzerker's Greaves OR Ionian Boots of Lucidity; the latter of which I reccommend, though both are applicable. As soon as you're level 6, pick up your Ultimate, and look around the map as to where it can be used properly. If an ally is low on HP and being chased, try to cast it near them whilst still hitting the enemy hero. If enemy heroes are low on HP and away from their lane turrets, cast it there. Otherwise, cast it near your original lane and continue laning, hitting enemy heroes if possible while still being safe. After farming/slaying enemy champs enough, pretty soon you should have enough for Sword of the Occult. Port back immediately and grab it.

Now the reason why I say Sword of the Occult is because though it only gives you the same amount of base damage as a simple Long Sword would, you can easily surprise your enemies with it after getting a few stacks. This is where you need to start being as aggressive as possible, as all of your allies should have the first rank of their Ultimate (at least!) and are able to use all of their abilities to help you slay some enemies.With the 1-2 combo of Aegis, then HsS, you should be doing some pretty impressive damage towards the midgame. Just be careful about doing so. You can tower dive once or twice with his passive block on, but don't be careless. Use the brush to your advantage OFTEN, as ganks will be much more effective than harrassing, in terms of getting more stacks for your SotO.

After getting that done and out of the way, you should have around 4-5 stacks, and well on your way to more. Continue being hyper aggressive, and try to farm when possible. This is so that you can get some base damage under your belt with B.F. Sword, and eventually an Infinity Edge. This will help ALOT during midgame as it will provide a base damage increase for you to stack more upon, and surprise your enemies even more. As soon as you can get it, do so. You should have somewhere between 6-9 SotO stacks by then, hopefully more. This means you'll have at least 250 damage. Once you are at this stage, not much will stand in your way without being torn to shreds. Just remember the combo and to play it safe when Skyfalling into enemy heroes, and shouldn't have many problems.

Once you get enough, I recommend Youmuu's Ghostblade as the next purchase. This is for four primary reasons. 1: Additional base damage. 2: More critical chance. 3: Additional (if you bought Lucidity Boots) Cooldown reduction, which he severely needs. And 4: Armor Penetration, since his Spearchuck and HsS both do physical damage, this will help them get through tank armor. It's relatively cheap and a useful item for this build as it provides several important key functions.

After that I also reccommend getting a Sunfire Cape, both for necessary additional HP, and an extra damage output when Heartseeker Striking. Panth on his own isn't much of a tank, so this helps him to not be so squishy. after that, it's up to you. I usually go with Black Cleaver, as it adds extra Attack Speed now, and will help you penetrate more tank armor, as well as being a decent amount of additional base damage. You could also go with another Sunfire Cape, or Warmog's if you need more HP. Frozen Mallet is also a somewhat good idea as the extra massive amount of HP, extra damage and slowing effect are very useful for him. This part is completely up to you though. You should see the enemy team trying to surrender by now, so anything after that is just icing on the cake.

"GIVE TO THEM NOTHING; TAKE FROM THEM, EVERYTHING!"
Good luck!