Remember:

Pantheon is a good help for jungle when it comes to invade or helping teammate junlger to finish buff faster.

Take a look on your carry, observe his/her play style if aggressive or play safe.

Also observe the playstyle of the enemy if its too aggressive play safe lure them to make a tower dive then punish them for their mistake.

Runes:
Most of the time, I use Press the attack rune to inflict more damage, but sometimes I use other runes such as:

Electrocute if your carry uses Press the Attack.

Conqueror if you like longer trades.

And Aftershock if you want to be more tanky and to help your ADC to push faster.

Core Items:

I use Umbral Glaive to destroy enemy wards faster

I also use Chempunk Chainsword to inflict grievous wounds so that healing is useless

Mythic Items:

Turbo Chemtank if the carry of the enemy team is AP type plus it gives a bust of movement speed and you want to be more offensive.

Shurelya is more of a testing item to me but if you want movement speed for your team while gaining mana regeneration, I think its a good choice.

Iron locket of solari is more of a defensive item and gives your carry to be the hero of your game.

Eclipse is a usual mythic item that I see with other pantheon support that I face in my ranked mathces due to its omnivamp and sheild.

Note:

This may not be the best guide for all of the players that uses Pantheon, But I just want to contribute to the game that we loved and help other players that are starting to use this champ.

I hope this guide may be worth your time and please if you see anything wrong with the guide fell free message me to improve myself in making guides like this.