Hello everyone ! This is my first actually tought out build so please tell me if i've done something wrong here. My build is about Rammus who is one of my favorite champions , and anyhow i hope this build is helpful to you.

Here are your skills as rammus:

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Spiked shell

Rammus gains additional damage as his shell becomes reinforced, converting 25% of his armor into attack damage.

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Powerball

Rammus accelerates in a ball towards his enemies, dealing 100 / 150 / 200 / 250 / 300 (+1 per Ability Power) magic damage and slowing targets by 20 / 25 / 30 / 35 / 40 % for 3 seconds.

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Defensive ball curl

Rammus goes into a defensive formation for 6 seconds, increasing his armor and magic resistance by 50 / 75 / 100 / 125 / 150 and returning 22 / 26 / 32 / 38 / 46 (+10% of total armor) magic damage to attackers.

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Puncturing taunt

Rammus taunts the unit into a reckless assault, reducing their armor by 10 / 15 / 20 / 25 / 30 and forcing them to attack Rammus for 1 / 1.5 / 2 / 2.5 / 3 seconds .

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Tremors

Rammus creates tremors beneath him for 8 seconds, dealing 65 / 130 / 195 (+0.3 per Ability Power) magic damage per second to nearby units and structures.

By my opinion the best summoner spells for Rammus areFlash andFortify.

Flashis good when you are trying to gank someone , for example:
You can charge your powerball , flash over a wall run them down and taunt them.
And also flash can be helpful if escaping , if your enemies have you surounded because they know you will try to powerball, then flash over them and start charging your powerball away.


Fortifycan be helpful when you are away from your lane or another lane's tower and it is getting pushed,then you can use your fortify to save you some time to get to the turret .
And you can combine Fortify and Teleport to make your lane practicaly unpushable and i very much recomend to have these both summoner spells if you are solo laning.

By my opinion those 2 moves are best for Rammus but you can take other summoner spells also - you can take exhaust,teleport and ghost other moves are rubbish for him.

As Rammus you should try getting a massive amount of armor to improve the damage of your defensive ball curl and so you can take a real hard beating.
But you also have to keep your MR ballanced too, around endgame your armor should be something from 300-400 and your MR atleast 100-150.

So mainly your items should be 4/6 armor and 2/6 should be MR.

Your core should be :
Mercruy's Treads -
They really help with their CC reduction and these boots are every tank's best friend.
Aegis of the Legion -
This item is very helpful to you and your teammates , it contains armor,hp and MR - all the things needed for any tank , and it helps your teammates with its unique aura giving Armor,AD and MR.
Thornmail -
Thornmail can be a very helpful item in the way that it is like a second defensive ball curl only, it's a item . So the way to say it , is - Thornmail + Defensive ball curl = DEATH to any AA. Plus it gives a nice amount of armor .

Other recomended items:
Force of Nature -
Force of nature is a realy nice item for it's nice amount of MR , nice HP regeneration and also the 8 % movement speed this item is a must , most of the time but when the enemy team has karthus , vlad or veigar then it's a better idea to get Banshee's Veil.
Banshee's Veil -
This item is a must if the enemy team contain's AP burst damage or something like TF or Karthus , this item is good agenst TF , Karthus , Veigar due to it's very nice unique ability - Blocks a negative spell every 45 seconds , if the enemy team doesn't have massive AP bursts or Karthus then it's a better idea to get Force of Nature.
Sunfire Cape -
Sunfire Cape is a really nice item it gives lots of hp and a decent amount of armor and it has a very nice unique ability - each second you deal 40 Magic damage to nearby enemies i recomend this item very much because in the end you do a massive amount of damage with thornmail,defensive ball curl,sunfire cape,tremors and add randuin's slow and all will perish.
Guardian Angel -
Guardian Angel can be helpful with it's unique ability - revives the wearer with 750 HP and 375 Mana , the effect can occur once every 5 minutes and also it gives a decent amount of armor and a bit of MR. i recomend to buy this item only as the last one.
Randuin's Omen
i strongly suggest that you buy this item here are it's effects :
+300 Health , +80 Armor , +25 Health Regen/5 sec. ,5% Cooldown Reduction and 20% chance on being hit to slow the attacker's movement and attack speed by 35% for 3 seconds. , slows movement speed and attack speed of surrounding units by 35% for 2 second + 0.5 seconds for each 100 Armor and Magic Resist. 60 second cooldown. and that is why i suggest this item it's just awesome and every tank needs this.