This is my first guide ever.
I got pantheon for free from Riot with his awesome Perseus Skin.
He's been my main since I started playing this game, and is my favorite champion.
So i decided to make a guide. This is how I play Pantheon.
I will be fixing and adding stuff in the future.

21-9-0

21 on offense for more damage; 3 on Sorcery for cooldown reduction.

9 on defense for a little bit of armor and magic resist early game.

Mark of Desolation: Obviously for Armour Penetration.
Glyph of Focus: For cooldown reduction. Pantheon is more of a burst champion. Cooldown reduction is necessary to use his abilities for frequently during the battle.
Seal of Evasion, Quintessence of Fortitude: For sustainability.

Flash: to escape or pursue your target. If your target is not in range of your Aegis of Zeonia, you can flash to get in the range.

Exhaust: Pantheon is kind of useless after using is Q-W-E-Q burst. So you will need to get Exhaust, Youmuu's Ghostblade, and Frozen Mallet to fight after the burst.

Other Options: You may use ignite to secure the kill, or cleanse and ghost for escape.

Core Items: Youmuu's Ghostblade, Frozen Mallet, Infinity Edge


Early to Mid Game Items:

Start with Meki Pendant and 2 Help Potions for mana regen and sustainability early game. Meki Pendant is needed to harass your enemy and last hit if necessary with your Spear Shot without running out of mana. But you still have to manage your mana since spamming your Spear Shot will result having no mana when needed. You will eventually sell this item later.

You should have enough money to get Brutalizer when you first recall. This is for cooldown reduction and some damage early game. This will be upgraded to Youmuu's Ghostblade.

You can get either Boots of Swiftness or Berserker's Greaves if you really want more attack speed.

Frozen Mallet is needed because pantheon is more of a squishy champion compared to other melee DPS Champions. This will give you HP, Attack Damage, and will slow your enemy.

Upgrade your Brutalizer to Youmuu's Ghostblade. Youmuu's Ghostblade with Frozen Mallet and Exhaust will let you deal damage after your burst. Bloodthirster will give you some life steal that will let you stay alive longer.


Late game Items:

At this point, you may sell Meki Pendant.

Heratseeker Strike's passive is 100% crit chance if enemy is below 15% of their max HP. Infinity edge will give you that crit damage to secure the kill and more crit chance during close combat with your core items.

If your enemy is Attack Damage heavy, then get Randuin's Omen. This will give you armor and HP. Its active is also useful during the battle, which slows enemy's attack speed and movement speed.
If your enemy is Ability Power heavy, then get Banshee's Vail or Force of Nature.

If your enemy is getting a lot of armor, then get last whisper for armor penetration.
If not, get Hexdrinker or Tiamat. (You will rarely get to this point)