Hi, my name is Michael. I am a jack of all trades and I can play any role decently well. Recently I've been playing a lot of Tahm support and I want to share with you everything I know. Why should you play Tahm? He has great supportive abilities and at the same time deals good damage, and is a great duelist. So let's get right into it!
*Note: Tahm hasn't fared well as of preseason 9 and I personally would not pick him, but if you really want to play Tahm, let's go!

Pros
*Extremely Tanky with grey health
*Can save allies from ANY CC
*Lots of early and late game damage with devour
*Great duelist!

Cons
*No real engage
*Slow!
*Ultimate is useless in combat
*Difficult to play if behind

Resolve tree
Guardian, because it is the supportive keystone in this tree.

Font of Life, because is it easy to use in lane, and the healing is very helpful! Hitting an enemy with your Q will mark them.
Conditioning. If you can just wait 10 min before you go ham, this rune is the best option and scales much better than the other two. In the end, this is just a personal preference and I see other people taking Second Wind as well.
Revitalize, because this synergizes perfectly with Kench- we have built-in healing and shielding, and we build plenty of shielding items which further increase the effectiveness of this mastery.

Other Keystones:
Grasp isn't good because it is very possible that they will have a ranged support, leaving you with very little opportunities to actually proc it.

is more viable, but my play style is more focused on keeping my adc alive and Aftershock doesn't help that. If you are playing aggressively, you should consider this keystone.

Maybe you've heard about Glacial Augment Tahm, but that is very hard to play effectively so I will not cover that in this guide.
Secondary Tree
Domination
Zombie ward, because this will greatly help with vision control as you basically get twice the wards! This is the reason the domination tree is viable.
Ingenious hunter, because we will be using at least 2 active items and we can drastically reduce the cooldowns just by getting assists!

Inspiration
These biscuits help lane go much smoother, especially if you are up against a poke support.
Approach Velocity, because you can get to your target faster just by hitting a simple Q. You won't be as easily kited! Tahm's glaring weakness is his slow speed but this helps fix that. Note that you also get speed moving towards movement speed impaired allies - perfect, so you can devour them!

Flash is not disputable, as it is too versatile and valuable to pass up. You can use this to flash devour to save an ally from a deadly situation, or flash Tongue Lash to finish a kill, or simply use flash as an escape.

You should take exhaust against a comp with assassins, or if the lane looks tough.
Ignite is when you want to crush an easy lane and snowball hard.

Build path varies greatly. There is not just one way to build and you will come up with your own preferred way to build. Here are some ideas though.

Core items
Eye of the Aspect. This item is an important source of income as a support and you should be building this almost every game.

Locket of the Iron Solari. This item was made for tank supports! This gives us the resistances we need and gives a shield to an ally in a fight. It's also really fun to keep track of how much damage was blocked. It makes me feel good inside.

Other items
Redemption This is a personal preference. I like this item due to the fact that if it is used optimally and hits many champions, it can turn around a team fight! Tip- use this when you see an ally reviving from something like GA. that way they revive with more health and keeps the enemies away.

Knight's Vow. I would only get this item if the opposing team is mainly AD. Otherwise I don't really like this item, as it's hard to even feel the effects.

Spirit Visage. Get this item if you are against 2 or 3 enemy AP carries, because this gives MR an the increased healing synergies greatly with your grey health passive.

You can't go wrong with buying tank items. Any and all tank items make you a better front line, which is part of Tahm's identity.

You generally want to be aggressive at early levels, normally levels 2-5. Your ultimate is useless in combat, so you might want to be a little safer at level 6. Tahm is very strong once he has all of his basic abilities. Look for extended trades, as your W deals insane amounts of damage (%Max hp magic damage to be exact) & suppresses your target! Remember to stay by your ADC to make sure Guardian procs if you are hit.

In teamfights. your duty is to protect your adc. If you go around trying to kill enemies, your adc left exposed will likely die so don't get distracted.

Generally, you want to be poking the enemy with your Q Tongue Lash. You need to assess they strength of their bot lane compared to yours. Do they have a weak lane like vayne and blitz? Then you need to be looking to devour an enemy champ and engage. It doesn't even matter who you can get. If they have a strong bot lane, such as lucian and brand, then your priority is to keep you and your adc alive until after laning phase. Place wards so that you won't get dived. Use control wards!!

This ability is what differentiates a good Tahm player from an average one. If you and your adc know you win a straight 2v2, ult to lane behind your enemies which forces a fight and usually ends up in 1 or 2 kills. If you win a fight in mid lane later in the game and your team signals for baron, ult immediately with someone to the baron and start it. If your adc recalled but you're still healthy, look to roam to mid lane. Getting your team ahead as Tahm is key because you can't 1v5.

-While your Q Tongue Lash is being cast, you can cast your W Devour for free(No mana cost)! You will not be able to devour champions in this way, but you can eat creeps and jungle monsters. W Devour will go on cooldown, however.
-After you Devour a champion, your passive stacks are gone. Therefore, in a duel, you should Q Tongue Lash a target with max stacks (to stun them) and Devour them after. There is no cooldown to your passive stacks; you can start adding stacks again immediately after you spit them out.
-Imagine your adc is coming back to lane, but won't reach lane in time to kill the cannon minion (or any minion). You should walk towards your adc, Q+W combo the desired minion, keep walking to your adc, and then spit out the cannon minion and your relic shield should kill the cannon minion and give your adc gold. You can also do this for your jungler if he is nearby. This trick also works if your adc is being zoned off cs, you can bring the cs to you adc!
-After you get the 500 gold from your support item, swap out trinket for a Sweeping Lens the next time you go to base because the support item gives you all the wards you need!

I would like to thank jhoijhoi for making this guide possible. Her coding template taught me everything I need to know for this guide.
And Thank YOU for reading my guide. Let me know if this guide helped!