The soul purpose of this guide is to play with the armor buff that Shatter(no idea how to link skills) obtains. The fact that you gain damage off of your W based on how much armor you have is just silly and the 400(guide doesn't show armor passive from shatter/shatter aura) armor you will have at end game stacks ridiculously well with the Randuin's Omen active use giving you a "4.4 second HUGE AoE slow" on top of the small aoe slow you get from the Gauntlet passive - not to mention you get to play with a Zhonya's in team fights assuming you initiate which results in wasted enemy aoe's(you go immune for 2 seconds while your allies gain 35 AD/AP, 50 armor, 30 mag res, 10 health/s and reduce enemy M-res by 20).

The AP from the Gauntlet, Zhonya's, and Abyssal is enough to keep the damage from your ulti/dazzle relevant and make your heal useful in late game.

The Abyssal is purchased because it's safe to assume that most teams aren't dumb enough to go full AD/Physical Damage compositions so some magic resist will be required if you are looking to be a slow moving bruiser from hell. If you want to go more "trololol I do no damage" then go ahead and purchase a Warmogs in there somewhere after your Gauntlet.

If for some reason your entire enemy team is actually full AD/Phys then go ahead and replace the Abyssal with a Rabaddon's and watch them flop under your Ulti/Shatter combo.

The Gauntlet is a key item in this build because:
Gives 40 AP (all of your skills benefit)
Gives 60 Armor (adds to tankyness and makes Shatter do more damage)
Gives 10% CR (spam skills more!)
Gives 500 Mana (honestly, you'd be oom all of the time without it + it gives an extra 10 damage to your auto attacks via passive)
Gives AoE Damage/Slow (This is a big deal if you flash in for an initiate like an idiot. Stop them in their tracks and hit them all for about 240 with your ulti up)

If your support is adamant about building the Runic, then build either a Warmog's or a GA. A Frozen Mallet would be silly with Exaust + Gauntlet slow + Randuin slow. Make your Jungler build the Frozen if it's needed.

If the team is heavy AP, then you're screwed *shrug*. But! There is a way to salvage this unfortunate random occurrence. You can drop your Zhonya's for either a Banshee's Veil or a Spirit Visage. I'd normally focus the Spirit Visage unless the enemy team has single target spells that have a stun/snare. Banshee's is amazing against champions like Veigar, since one of his abilities is delayed(walk out of it) and the other 2 are single target. It all depends on who the primary and secondary damage dealers are. The Spirit will give you some decent M-res + HP + 20% CR and will make your heals on yourself amazing.

Does Rylai's Crystal Scepter slow effect work off of Taric's passive as it does magic damage?
No, it does not :(

You can buy armor quints instead, but the soft armor cap at 400 doesn't scale well with YOUR physical damage reduction. I had 480 in a game and only got 2% more reduc than I did at 400. This guide will get you there (or at least really close)

Movement quints wouldn't be a bad idea however - assuming that you are amazing at last hitting with one of the slowest attacking champions in the game. Doing so would put you at around 403 move speed after Tabi which is a respectable chase rate

Greater Quintessences of Attack Speed.
- I normally don't last hit since I am either in the jungle or supporting bottom lane, so having these really makes the world of difference for me. To each their own

Greater Glyphs of Armor
- Why, you ask? Wouldn't Scaling Magic Resist be more efficient? Listen herp, you are building a Runic Bulwark which is going to give you 60 magic resist(yes the base stats + aura stack) for mid game and the abyssal will give you another 45 in late game.