Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This build goes mostly off of Randuin's Omen's active. An AoE slow/attack speed debuff can turn the tide of a team fight in an instant. With all of these items and Defensive Ball Curl active, it will last for 6 seconds, which is just long enough for your team to mop up your enemies.
Your summoner spell choices should focus on getting in the middle of the battle and getting out if things get rough. Cleanse is a great choice for this, allowing you to freely move about while you find a good position for the active. Ghost is good too, but Flash could be substituted if you prefer.
0/21/9. Grab the upgrade for Cleanse and Ghost.
Defensive Ball Curl is the priority, followed by Tremors, then Puncturing Taunt, and finally Powerball. One level of Powerball in the beginning is what I have been using, but I've noticed that the slow is a lot better at level two, so if you need more slow (which you probably won't after Randuin's) get another level. Use Tremors as a DPS tool or to spank towers. When you go to attack a tower, hit Defensive Ball Curl and Tremors to take it down fast.
Like I said before, Randuin's Omen is your priority, so it comes first. Start with either a Ruby Crystal to start, or Cloth Armor and several Health Potions. Work on getting Heart of Gold first, and then Warden's Mail to finish it off. At any time in between, you can get Ninja Tabi if you need to move faster. After boots comes the Frozen Heart. More mana (meaning you can stay out longer), along with the fantastic armor boost, attack speed debuff, and cooldown reduction make Frozen Heart a great choice for ANY Rammus build. After all of that, I get a Madred's Bloodrazor. This may seem a little unconventional, but it works wonders. With his passive, Rammus gets a pretty decent bonus to his damage. To take full advantage of that, a little attack speed and even more damage and armor will do wonders. I've also thought about using Wit's End for caster-heavy teams as well. If you don't like either, I GUESS you could get Thornmail or a Sunfire Cape... however, be aware that Thornmail will not reflect a lot of damage with the attack speed debuffs of this build. That's why I ignore it. I haven't gotten farther than this, but you have a few options. Guardian Angel will give you MORE armor (like you need it) and some magic resist to boot. Force of Nature or Banshee's Veil is a good choice against caster heavy teams. If you really want to conform, you can get a couple Sunfire Capes, but another goal of mine with this build was to make one without any Sunfire Capes that was just as viable. I think I succeeded.
Take top or bottom, don't go mid (hopefully I don't have to tell you that, but I thought I'd mention it). You can take a lot of hits early game, so don't be TOO defensive. If you're laning with a squishy partner, keep them safe! Most of the squishy champions are DPSers who will suck if they die too much.
You can start to move around and help lanes once you have Randuin's and your boots. If team fights are starting, go be an initiator. You can sustain some decent damage at this point. When you are in the thick of it surrounded by enemies, hit Randuin's active to turn the tide of the battle. Combined with Tremors and Frozen heart, the enemy team will either scatter or fall in no time.
Now you can take a serious beating and live to tell the tale. Keep using the active. It's cooldown is fairly low, so it can be use frequently. Keep pushes going and take down towers as much as possible.
So as with any Rammus build, you won't get a lot of kills, but assists are going to happen very often if you play right. Try it, rate, comment. Tell me how you did, I love feedback.