Well.. this is my go at making a builg/guide for a champion, which just happens to be that big, nasty and annoying frog a.k.a. Tahm Kench, so its your job and duty to play him as described ... especially the nasty part , becouse the licking never stops :)
I didn't make a specific role for him, because you can take this build bot(as support), solo top, or mid. Becouse the item setup brings the most of his kit, its not about the lane you take, but more about what you do as Tench Frog. Its all about the style, so you have to pull the maximum swagga out of this amphibian king of the river.

Little to none words lost for runes, just to summorise it:
-AD MARKS - you need to farm with Kahm, and honestly thats a pain in the *** without ad items before you get your ult (passive (more about his R later)) , and becouse I don't wanna waste gold for items that A) will be sold later for a ripoff price, or B) wasting slots for items which don't benificiate you as much as others
-ARMOR SEALS - self explainatory (you are off tank , so better rune like it)
-MR GHLYPS - same as above
*its a fact that that choice is the most viable on as it goes for deffensive stats from runes
-AP QUINT - you'll be building offtank, AP OFFTANK .. so its a must some ap comes from somewhere else than your items (important for very earlygame and your Q and W combo harras at that point)

Same as goes for runes, I won't lose too much time explaining why these masteries, becouse it comes more less self explainatory.
When picking masteries I looked to add some stats which help Tahm throughout the game, focusing on defence as you are an offtank, and adding some cd and movement speed.

- many uses.... main thing to remember: When you devour your ally, you can use flash while he is still in your belly - this makes it a perfect spell for The River King, becouse you can save a teammate almost every time you devour them

- (never say no to true damage :D) provides much needed offence, don't be afraid to use it

These two spells are my favourite picks for the way I am building Tench they give you mobility and dmg output, which is what he lacks early on, and only ads up when you are getting items

Follow the item path I've shown at the beggining. :) (sell the 2 doran's rings only if u need aditional place for items, becouse they are worth keeping till the end of midgame)
*ALSO: at the first trip back try to have enough gold for 1)Boots of speed, 2)2nd Dorans ring and Bami's Cinder (this is perfect first backtrip, aim for this - this means you should focus mainly on farming rather than fighting very early on)

Every item makes you better and better at the time you get it in the game. From farming aspect (building Sunfire Sunfire Aegis (same goes for Bami's Cinder)), healing through your e passive (building Spirit Visage (also the Spectre's Cowl healing, after intaking magic dmg, shouldn't be ignored)), making two Giant's Belts is the bread and butter for you ult's passive effect(and final itmes only add some extra hp stats, so thats also a jolly thing if you think about it for half a secound)

Where I would like to lose a few more words is the 2 ap itmes in this build-> RYLAI'S CRYSTAL SCEPTER & LUDEN'S ECHO
--> The main reason you build Rylai's Crystal Scepter is becouse your ults passive, which does ap dmg on hit eaqual to a portion of additional hp. That being said some would still argue you have pretty nasty slow with your Q to land that extra one or 2 hits before you get 3 stacks of your passive on the enemy. All nice and well, but we must take into the consideration that you might miss Q - the worst case scenario, you'll have the slow from basics and so your enemies will sick to you through that single target ability (remember: passive from ult). But the main reason you build it lies in your main engage combo: aa(autoattack)-Q-aa , that being said, eventho your Q has a nice range, the ability is a little delayed and its not that hard to dodge if you don't predict where will they try to move to avoid your ''skillshot'' - basicly its your tounge , but lets not drift away from the topic now :D ... with the slow from first autoattack you will also land that tounge easier. And the most important part: YOU BENEFIT FROM THE HP AND AP, so its a no brainer.
--> Luden's Tempest gives you movement speed, which you kinda lack (and for that reason you also build Aether Wisp right after you get your fancy boots) , it gives you a nice portion of ap, and its passive works crazily well with either your Q or W (firstily for easy wave clear later when game progresses, and in the lategame when you poke before the Teamfight (but more about that at the end of the Skillz chpt.)).

As I said at the top of the build, last item is solely dependent of how you, your team and the enemies are doing............................................................................................................................................................

** ~> LAST ITEM, FROM MOST VIABLE TO THE LEAST
Sterak's Gage - a very good final item for Kench IMO, it gives you quite dope passive, and stats themselves aren't that bad aswell {pros: passives shield, hp and also little ad to boost on hit dmg(one via ultimates passive one directly) / cons: you don't build any ad with this build so the ad boost of stats (25% of base dmg) ain't that much, but allaround I like this item a lot}
Warmog's Armor - {pros: good for your E, ultimates passive feeds of it like crazy / cons: no deffensive stats, no ap}
Rabadon's Deathcap - take this item if you are doing really good, so consider it with causion {pros: more ap dmg output / cons: no defensive stats, no health to convert into ultimates passive}
Liandry's Torment - build it if enemy team has more than one tanky building champions {pros: hp, ap, burn passive / cons: hp stats don't increase a lot, ap same, but is good for burning through the magic resist}
Banshee's Veil - for boosting MR stats, sexy passive
Thornmail - take this only if their ad carry got fed, pure armor item, your kit doesn't actuall benifit a lot from this item, still viable

I'll just give some general tips for your skillset, solely 'cuz I belive you have to try a champ out and learn about its skills in the game (your preception makes the way you understand each individual skill, and that takes some different scenarios in which you find yourself through-out the game.) So I'll jsut summarize with one sentance or two each skill, and what you should do to make most of it.

You max out Q first, secoundly W and E last (but take another point at one point of the game), also a poin in ult when possible. Reason you max Q first is solely in that you are left very vanurable after using your W (due the around 10s wait for CD). Also the dmg scaling on your Q is ridiculisly good, which is only another plus. Now lets talk a little about the skills alone:

0.)) passive: its the setup for special effects from Q(when stacked you will stun them for 1,5s) and W(nomnom effect).
*note to self that it lasts 7s, which is one of the longest marks in the game and that lets you land all stacks easier

tongue leash 1.)Q) Tongue Leash - it does crazy dmg and slows crazily too(70% when maxed)
*tip: you can use W in the middle of casting Q on minion, to directly devour(W) it from range

//Regurgitate 2.)W) Devour/Regurgitate - 3 different uses of this ability, you can use it to a)eat a minion and then spit it as aoe skillshot, b)eat your ally(more about at the bottom of this chapter), c)when you get 3 stacks on an enemy champion you can eat them too.
*if you devour an ally you also gain movement speed, whereas if you do that to an enemy you get slowed by 95%, so think where, on whom and when to use it
REMEMBER: this ability has multiple uses and it makes Tahm what he is, when you devour somebody you basicly remove them from the game for a short period of time, which can be a decisive thing if done correctly - allways try to help your allys

3.)E) Thick Skin - you'll need to play a few games to master your decisions, wether use it or wait for the healing effect(passive). This ability becomes a very frustrating thing once you reach lategame - when used correctly you can absorb(tank) enormous amount of damage for your team (if you have.. lets say 3,5k hp and you engage, you intake 3k dmg, and then you activate your E. which leaves you with 3k barrier/shield (lasts 6s) - just make sure you get focused instead of carries).
The passive... I really don't know how much love can I give out to this passive effect :) It heals you up to 44% of the intaken damage, in other words almost 1/2 of dmg done to you is recovered back (over a short period of time). That. Is. Insane. And this makes this bullfrog such a beast in lane, even if you could be poked instantly, 50% of that damage means nothing to you, and you don't have to worry about trades with the laner(s), you can farm without even flitching when someone tries to deny it from you, which makes it perfect for the build intended. It allows you to do the most important thing you can do, which is farm, farm and FARM again. Don't forget that you need a few items to reach your true potential, and most of the gold earned for these items comes from... yes, it comes from those minions you are trying to lasthit, so pay attention and try to reach as higher number of cs till midgame as possible.

4.)R) Abyssal Voyage - This ability has also an active and passive. Active teleports you and also one of your allys(if he clicks on you during the channel of your ult(active)), which is an ok active, nothing great or overpowered about it, becouse its long channel and also notifying enemies that you come(with a ping on the minimap), but the passive effect is another thing alltogether. It allows you that a percantage of your hp is done as on hit magic dmg effect (it scales with the lvls 4/6/8% of aditional hp (20dmg is the base)), that basicly means the more tanky you go the more your autoattacks hurt :)

*take that into account becouse even if you don't follow the build I'm making myself for Tahm Kench, you can still try him as fulltank role. Thats also worth a try (I didn't make an example of that build here, becouse the items you buy depend on who from the enemy team is getting fed(farm, kills..) and you simply just counter them(if the champ is ad you buy hp/armor, hf/mr for ap carries) and the amount of aditional hp going into on hit dmg will reach up to 300on hit, maybe you add titanic hydra and there you have tons of ''disquised'' dmg potential).

YOUR COMBOZ:

1.) the main thing you should do while 1v1, aa-Q-aa (the q resets autoatack - this is the fastes way you can stack the passive) and after that have a good meal :D (so there is no confusion: you use your W ability)
2.) poke-combo: W a minion first, then you hit your Q after that you spit the minion at your target (Q applies slow, so W is easier to hit. you use your Q before W with another reason: if w isn't maxed out and if you don't have enough ap, you will hit with your tounge minion that you spit if you do this combo in rewerse) +when you hit them with Tounge Lash it does, if you build correctly, quite a lot of dmg so your enemy might even use flash to escape the certain death- here comes W to much use, as its range is larger than Q's and the skillshot ''flies'' faster than your Q.

YOUR PURPOSE: basicly you are the spunge that takes damage, you are creepy guy that licks enemies to despair, and you are the saviour which comes out of nowhere to devour your overextending teammates and carrying them to safety :)
p.s. the last part is one of the most important aspect of your play, becouse its uniqueness can catch the enemy team off guard, they might overdo theyir fight to get your carry who is on killing spree, but you know what, you stick them the finger, you eat your carry and then you run away, flash away.. basicly do wahtever you want to do, to save that teammate. This is fun if you do it correctly, and these play might turn the tables if you are losing.

I'd like to apologise if I had made a few typos (if that made the read frustrating), but I'll not be making any amends to fix that... :)
So that was my guide, I tried to keep it as simple and logical as I could, also I wrote about things I think matter most, but the general idea was to make a quick guide to success, I really hope it helped someone and that you'd get the general idea of playstyle, as that was my intention (other than introducing the way I play the frog this guide was dedicated to) while making it.f you spare a minute please comment, if not then rate it, I'd appreciate it greatly, solely from the response aspect, so that I see what I'm doing alright and what should be changed.

THANKS AGAIN FOR THE READ, AND MIGHT YOU REIGN AS THE RIVER KING NEVER END !! ;)