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Cho'Gath is one of the best tanks in all of league of legends. His ability to farm for ever in early game can make him just about unstoppable in late game. He can be played in two ways. He either be a straight tank or he can be built into a tanky caster. I personally choose to play him as a heavy tank. I have found that when played this way it is almost impossible to die. You can walk straight into a 1 v 5 and still get out of it alive.
For runes i just go with a standard set of tank runes. I get armor reds, health regen yellows, magic resist blues, and flat health quints. These runes make him a little more durable in early game which is the only time you are weak enough to be killed easily.
For masteries i go with a 0-21-9 set up with masteries in all of the major defense areas and in utility i take the health and mana regen boost along with the increased experience gain and the extra gold per 10. The increased experience gain is extremely important on cho'gath because he is one of the best 1 v 2 laners in the game. He can easily do this based on his passive. For every unit he kills he regains some health and mana. This allows him to stay in his lane for a very long time even if it is being pushed very hard. You will often find yourself out leveling the mid since you can just stick around for so long,
I start with a regrowth pendant which later builds into a warmongs which is probalby the most important item you can get on cho'gath. In conjunction with 6 stacks on his ult. He will have well over 4k health. This and the combination of a high amount of armor and magic resist is what makes this style of cho'gath next to impossible to kill. I get boots of swiftness because without them cho is very very slow. The final 4 items give high amounts of magi resist and armor along with some very useful passives. The guardian angle makes you even harder to kill. Banshees gives some health and mana along with magic resist and of course its passive is very useful. The health regen and magic resist of the force of nature is great but the bonus movement speed might be even more useful in the late game to catch up to those quick carries o the opposing team. The final item i build is a thornmail. This item gives tons of armor and it provides a huge amount of lols at the end of the game. Can just watch the opposing teams physical damage champions heath drop like a rock as the try to kill you. remember you have over 4k health so if an enemy champion tries to kill you one on one he will take 1200 plus damage just from thornmails passive alone.
I max vorpal spikes first since for the first at least 10 levels all you will be doing is farming. Vorpal spikes allow you to get more than one minion kill per attack allowing you to regen health an mana faster. Then i max rupture and the feral scream. This two abilities are more useful in later game team fights.
I choose flash and ghost as my summoner spells to improve cho'gaths early game escape capabilities. He can be killed fairly easy in early game if you aren't careful.
In a team fight obviously you should always go in first. With 6 stacks on your ult. you will be so large that it can be hard for the opposing team to even click on another ally. This is one of the reasons he is such a good tank. Once you initiate the first thing you should do is throw a feral scream into the middle of your enemies followed quickly by a rupture. if you manage to get both of these of on most of you enemies the fight is already over for your opponents since your team has had several seconds to attack them without them having anything to do about it.
So basically that is how i play cho'gath. As you can see this build makes you the ultimate tank. Give it a try. If you like it rate it and if there are any questions feel free to leave them in the comments.