I'm a Pantheon OTP with 400k points on him and playing with him for over a year.
I made this guide because I was bored and I felt like I knew a lot about Pantheon and I wanted to share my 1.5 year experience with him and provide the best info that I have. I'm playing on EUNE and this year I went Silver in SD and Gold in FLEX. Now I don't really play ranked but I want to climb to be honest.

This Guide focuses on my experience with Pantheon and his Items, starting items, first back, core and situational items. His Matchups and a little bit about his combos and the champion himself.

Pantheon is a fighter caster champion. As the years passed his identity is kinda weird. He is considered an assassin, a fighter, but not a bruiser, constantly changing lanes. He is most common in the Toplane, but he is better at Mid. He can be played as a Jungler and as a Support. You could go ADC but at your own risk. Pantheon is an Anti-carry meaning that your role as a Pantheon is to prevent the enemy carry from actually carring the game. You could also try to carry a game if possible. Pantheon doesn't require a lot of mechanics because he is easy mechanicaly, but he requires good macro knowledge to beat your opponents. About lanes, Pantheon is better for me at Top because he is a better blind pick in there, while at Mid he is better as a counter pick since he counters most of the AD assassins, while he gets countered by mages in the Mid lane. In lane you try to bully the enemy laner early by poking. Also when laning you always want to look if the opponent has Bone Plating, it can block all your damage when engaging.

:
Easy to pick and play.
Great at oneshoting targets.
Big Roam potential to help.
Amazing early game.
You can solo carry games on your own.
If your early game goes well, your mid and late game should go just as good.

:
Falls of late game if behind.
If your early game is not good nor your late would be.
Kinda immobile.
Vulnerable to hard CC.
Falls of at 6 because of his non 1v1 Ultimate.
You just need to survive till Mid-Game.

Abilities:

Passive: Every Auto Attack or ability you press, you gain one stack. At 5 stacks your abilities are empowered, except your ultimate.

: It has 2 uses, the first one is a stab and the second one is the charged one and it becomes a skillshot.
Your Q stab deals the same damage to all enemies hit, your charged Q deals normal damage but if it hits more targets, it deals 50% less.
Also note that your Q executes enemies below 20%, also it deals 30% reduced damage to minions and 5% increased damage in jungle monsters. Your empowered Q deals more damage.

: Your W will stun an enemy for 1 second, it can be used on minions or jungle monsters. (It's a leap).
Your empowered W will stun for 1 second and it will apply 3 Auto Attacks for the duration (On-Hit Effect). Trick: You can use empowered W and stun him but cancel your 3 Auto Attacks and use them later.

: Your E will mitigate damage from a single direction. Your shield must face the opponent in order to prevent your HP lowering, if you face against him, you will take the damage normally. Empowered E gives you 60% movement speed for 1.5 seconds. Also your E has a second cast which Pantheon slams the shield to the ground and deals damage.

Lastly your is not a 1v1 Ultimate but a roam one. You can use your Ultimate to roam to different lanes to help other teammates or escape a fight you cannot win. It gives you 10% ArmorPen at level 6, 20% at level 11 and 30% at level 16. Your Ultimate needs 4 seconds to succesfully land. 2 seconds on the ground and 2 seconds while hitting the ground. Your Ultimate has a spear in the middle, if enemies get hit by it they will get 50% slowed for 2 seconds. While the edge of your Ultimate and the area around your spear will deal magic damage.

is your way to go rune, since you will be using it in most of your games. This rune after the first 3 Auto Attacks (you can proc it with your W), for the next 6 seconds you deal more damage to all enemies and this applies for your teammates too.

You take when you are facing tanks and heavy juggernauts. You can still use .

For your secondary tree you take most of the times , because it gives you sustain which you lack.

You can choose and depending on the opponent and the play style you prefer.

You take everytime , since it's your most valuable first rune page.

You want to take because it helps you with your mana problems, but you can take to help you at fights.

For Legends, you want to take as any form of CC can cancel your all-ins. You can also take if you feel pretty confident against your laner.

Lastly you take if your opponent is squishier than you and can burst him easily. Take if the enemy laner is a tank or he has more max health than you. And for last you take if you think that you will be less HP than your opponent when you all-in.

This was my first guide and it was a lot of work.
Anyways, I want to thanks everyone that even took a little of his time to click and view my guide.

Thanks again, and I will see you next time. Have a great day!