This is my off-tank Pirate-Pantheon build. This guide will turn you into a gank machine. Once you hit level 6 the other team should fear you jumping onto their tower and destroying them, while you block turret shots.

I created this build, because I felt Pantheon is useless late game if you build him dps. He is too squishy and doesn't deal enough damage (late-game) to be the team's carry. But, that doesn't mean he can't get kills. You should play him as the initiator and clean up crew.

Pros:
High damage early game.
Low CD ranged attack and stun
Good ganker.
Ultimate can help teammates across the map, and gank!
Great initiation into team fights with ult.
Hard to kill late game, and is still a threat.
Looks awesome.

Cons:
Damage goes lower and lower the farther you go into the game. (Still stays decently high.)
He can be outlaned by skilled AP champions.
No escape if you are ganked, unless you have a summoner spell or Ultimate up.
Ultimate has to channel, so the enemy sees it coming (could be pro)
You WILL get targeted in team fights.

After attacking or casting spells 4 times, Pantheon will block the next normal attack that deals more than 40 damage to him.
This allows Pantheon to block towers and ALL auto attacks. Charge this up by hitting minions in lane. Make sure to have the proc before you go in for a tower dive.

Pantheon hurls his spear at an opponent, dealing 65 / 105 / 145 / 185 / 225 + 140% of his bonus attack in physical damage. If Heartseeker Strike has been leveled up once, Spear Shot will deal 150% damage against opponents whose current health is below 15% of their max health.
Spear shot is your harass during the laning phase and throughout the game, we max this for maximum damage, because it has a shorter cooldown and more damage than Heartseeker. Also it is a guaranteed damage spell, meaning you just click the enemy, no aiming required.

Pantheon leaps at the target enemy champion and deals 50 / 75 / 100 / 125 / 150 (+100% of ability power) magic damage and stuns them for 1 second. Pantheon also instantly refreshes his Aegis Protection.
Your stun and gap closer, use this after spear shot to get in range for Heartseeker and auto attacks.[/i]
This is pantheon's gap closer and his mini-stun. This gets you in range for auto attacks and Heartseeker.


Passive: Pantheon gains 100% critical strike chance against targets below 15% of their max health.
Active: Pantheon channels for 0.75 seconds and deals 3 swift strikes in a cone in front of him. Each strike deals 13 / 23 / 33 / 43 / 53 (+60% bonus attack damage) physical damage, and they deal double damage to champions.
It does nice damage, but requires your opponent to stay in it's range for the full duration. Pantheon's skills flow together, because his stun will allow you to land this easily.

Pantheon gathers his strength channeling for 2 seconds and then leaps high in to the air, crashing down at the target area 1.5 seconds later. Deals 400 / 700 / 1000 (+100% of ability power) magic damage to units at the center (down to 50% at the edge) and slows them by 35% for 1 second. If the channeling is cancelled, the cooldown is reduced to 10 seconds.
His bread and butter, although it was nerfed, you can still use this to gank the hell out of anyone trying to push a lane. Read below in the guide for tips on using this.

I build him Attack Speed and Armor Pen. The attack speed helps, because he doesn't get any attack speed items, and it helps proc his spear's passive, which does 150% damage if the target is below 15%. So you are a tank killer when they get low. You should be able to finish off people pretty easy with this passive. If they build HP you will proc when they get around 600 HP... Riven is going to hate you.

If you want you can get flat AD runes, for domination early game. I don't think they are needed, because Pantheon already destroys most mids.

You can do anything with Pantheon's masteries. If you want, you can throw in the AP in offense, so his W does more damage... You can go straight defense for the extra beef. I like to go 21/0/9 for the nice utilities and buff duration. Although 21/9/0 is just as viable.

This is all personal preference.

If you are going against someone you KNOW you will dominate get Doran's Blade. But, if you think you will be harassed and sent away from minions get Boots and 3 Pots, for quicker Cutlass and health regen.

This item is what separates the other Pantheon builds from mine. Other people choose to build Frozen Mallet or The Brutalizer first, but I believe they lack punch on Pantheon. Although Ionian Boots and The Brutalizer makes your speer have a 3 second cool down, it doesn't build into anything useful on Pantheon. It's an early game item, and this guide is about destroying throughout the whole game. Frozen Mallet takes a while to build and you could already have your Cutlass, so let's compare.

Cutlass:

Ranged
Deals 150 magic damage
Slows more than the Frozen Mallet
Attack Damage
Life Steal
Cheaper

Mallet:

Procs on every auto attack (Pantheon shouldn't be auto attacking that much, unless buying time for another spear.)
Slows (Less than cutlass)
Gives HP (You end up buying this later when you actually need the HP more.)
Attack Damage

I might be bias, but getting the Cutlass before the Frozen Mallet is much better.
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After your Cutlass you should be getting to level 6 or already be there. You should start looking at the mini-map for when lanes are pushed. See below for the skill order when ganking.

You can get any boots you want, but Ionion Boots help with your ult cool down and your spear shot... well everything in your arsenal expect your Cutlass.

Then I get the Frozen Mallet, get a Giant's Belt first for the HP, and you don't need the 25% chance to slow, because you have your cutlass with it's 60 second cool down. The slow doesn't really help until you upgrade it to a Mallet. Once you get the Frozen Mallet no one will be able to escape your slows and stuns. You will still do VERY nice damage in 1v1s.

When you start out, get spear toss, because it is CRAZY damage when you are low levels. If you have an enemy that is squishy, you can zone them the whole game with spear toss. Also take advantage of your block if you are against a right clicker. Make sure to always be charging it back up by attacking minions. It can block gangplanks parlay and Irelia's dash btw. Some other skills as well. Always try to get middle lane, because you can get better range for top and bottom ganks. Once you get your cutlass you should go for the kill on your opponent. Throw a spear, jump on them with stun, use heartseeker, and then cutlass. Now they should be low hp and slowed for the next 3 seconds. You can either ignite them or auto attack and spear until they die. It depends on where you are in lane. Once you hit 6 start ganking.

The combo for ganking is as follows:
Grand Skyfall>Spear Shot> Aegis of Zeonia>Heartseeker Strike>Cutlass>Auto Attacks and Spears until they die.
Make sure to ult behind them, so that they are forced to engage onto your ally, or run into your high-damage ultimate. They will usually try to run into the edge of your ultimate and towards their tower. If they flash away or somehow get past your ult, then you can use Aegis first, and then Cutlass just to be safe. But the above combo is for maximum dps.

Make sure to aim the ult smart. In team fights it's good to initiate slightly behind and slightly on top of them, so they have to decide to run and take damage from your team or fight and you will hit their ranged champs, putting you in position to kill their carries. Also it allows your team to stun/snare them under your ult.

You need to be ganking the whole game with your jungler. It's EXTREMELY powerful to 3v1 top with the help of your jungler. Be aware that if you go into the bushes mid, the other player will most likely warn top or bottom respectively.

Hope you enjoy this build, and please give support even if it's criticism.

Also, I looked around and saw this build: http://www.mobafire.com/league-of-legends/build/avast-bilgewater-pantheon-24306
I did not know of this until AFTER I tried this in-game a couple of times. Besides, his build is focusing on dps, which I disagree with on Pantheon.