This build revolves mainly around the terrible trio, Sunfire Cape + Abyssal Scepter + Thornmail = pain. Here's how it works:
-The sunfire cape gives you health and armor for your tankiness, but the passive aura's damage over time helps you be effective in farming and teamfights without relying solely on your probably low damaging auto attacks and abilities.
-The abyssal scepter gives you magic resist for tankiness (since you already have health and armor from sunfire) and the ability power might help you with some of your abilities if you scale with AP, like Alistar does. The passive aura on this item reduces enemy magic resist, making your abilities pack even more of a punch (as well as those of your allies) AND it helps increase the damage over time per second of sunfire cape.
-Thornmail gives you a boatload of armor to negate the damage of AD carries. The passive on this returns a percentage of the physical damage done to you back to the attacker as magic damage, which will be amplified by abyssal scepter's passive as well.

The rest of the build:
-Boots of swiftness will help you stay on your enemies and escape when you need to. Other boots can work well too, but these are generally the ones I get to deal with Alistar's lack of speed.
-Warmog's Armor is a tank's best friend. After you own the other items listed above, it will be easy to farm up and fill out this item quickly. All the extra health and health regen will make you nearly unbreakable.
-Force of Nature will give you all the magic resist you need. Also, it will give you a great amount of health regen, making you a real threat to leave alone for too long.

Overall point:
You will have a lot of health, armor, and magic resist, as well as tons of damage. The longer you're in the fight, the more damage you do. If the enemy focusses you, they will lose a lot of time in the teamfight trying to finish you off AND all three of your terrible trio items will be able to do their job as you're getting focussed. Also, if the team ignores you, although that is a better choice, you can still use abyssal scepter and sunfire cape to rough them up while they try to get the rest of your team. The best bet to counter this build is for the enemy to build a lot of magic resist and to keep their distance in order to avoid your spells and sunfire cape.

That's all I really have to say for this build. Use whatever runes or masteries you see fit. This is just a basic idea. Alter it as needed for your personal preferences and champion choices. It's an expesnive build so the longer you can make the game last the easier it will be to complete and use to the fullest. Keep that in mind when trying this out. Thank you,that is all.