It still needs a lot of work, but I'm going to UPvote because it helps when your guide is still growing(^^). You should give more items options(or create a new build) because the selected items are very situational.

Thanks for your reply :)

I can see your point. I really find that the gold runes work for me in the laning phase.Its just that little extra cash to have my core items a little quicker. The AP and Magic pen. Marks is something i've tried before, but didn't seem to add much. With this build I'll have my core items a little quicker than the opposing teams Tahm and seem to be able to snowball that potential a little better. Since I'm consistently able to buy tier 2 or higher items. However ill swap it again for a couple of games to your suggestion and I might come back on my answer.

On the ancient coin: I used to start relic shield a lot. The passive of face of the mountain is really good and the active scales with maximum health. Definitely a viable start. However the ancient coin also works really well on Tahm. It gives sustain and you passively get just a little bit more gold. I sell it when I am almost full build and need the inventory space. So that is ~40 minutes in game.

This build is aiming for Tahm to be like an extra 'top' laner worth in items late game. Also the reason why I usually don't build sightstone but upgrade the ward. The extra cash helps to get there. If I'd go for a normal support build, aiming for utility over damage, i'd go different runes and masteries. Those build allready exist, so I saw no point in writing that. :)

Thanks again for the feedback, ill definitely give it another try.

Gold runes are useless, runes' main purpose is to serve the early game phase, later on, their stats are insignificant compare to the stats from items. 6 gp10 seals + 3 gp10 quints give 27 gold/min , which mean for the fist 15 mins, which usually is the duration of your laning phase , you'll earn 405 gold . Whereas 6 Mr pen marks (worth ~180 gold) + 3 ap quints (326 gold) give you ~500 gold worth of stats right at the beginning.

Ancient coin start is also sub-optimal, you also seem to sell the ancient coin afterward for another item, which makes it even more of a waste. Kench works well with health stats, why not go for relic shield? The passive can heal your adc as well.