Hello people of Mobafire, and welcome to my Pantheon guide! This guide will hopefully give you success with this fabulous Spartan, and I hope you enjoy it most of all :)
Anyway, I would like to say that I bought Pantheon at a very early level (and skill level). At first, I didn't do so hot. I would always end up with something like 2/4/10 or something like that, and it really frustrated me. So, I took a long break from him.
After I decided to try him again (about 15 levels later), I absolutely R.A.P.E.D. 21+ kills and no deaths straight. I felt so amazing, so strong!
But anyway, that's besides the point. In this guide I will cover the different parts and why I chose what.

Okay, I decided to talk about the items first because you are probably thinking, "WTF? Hybrid items for Pantheon? You're stupid, Dubz."
I don't blame you.
But, I will explain. Heartseeker Strike, even though it scales off of AD, counts as a spell-cast. So, if you have spell vamp on a high damage and coned ability, you are going to heal yourself a lot and still do a lot of damage. Plus, Aegis of Zeonia scales off of Ability Power, so you can do moderate damage AND stun them. Sounds good to me!

Here I will explain the runes used here.

Greater Mark Of Desolation - Some armor pen for your AD side of the hybrid build.

Greater Mark of Insight - Some magic pen for your AP side of the hybrid build.

Greater Seal of Replenishment - Pantheon is uber mana hungry, so we get some mana regen to help with that.

Greater Seal of Potency - Ability power for your AP side of the hybrid build (obviously).

Greater Glyph of Focus - Cooldown Reduction, so you can pump your abilities out faster.

Greater Quint of Desolation - Some more armor pen for your AD side of the hybrid build.

Greater Quint Of Insight - Some more magic pen for your AP side of the hybrid build.

Since Pantheon is an assassin, you quickly use your abilities to burst down the enemy team's carry. In this section, I will explain your skill sequence to use for completing your job.
Oh, and by the way, I should say that you want to go for the squishy champions, because you won't do much to tanks, even with a hybrid build.

Basically, your skill sequence should look like this, and make sure your target has a long way to go before escaping:

R
W
E
Q
W
E
Q

That combo should take out your target, because it always works for me. Sometimes they will be already dead halfway in this skill combo! Since nothing else is to be said, let's move on.

Well, I don't have much else to say, except I hope you find this build helpful and effective!