Somehow this is making me sad :( All I can say is that these builds work fine but I don't like the way how Riot is raping my favorite tank. Tha old good sunfire stacking wakes up so many good memories on me.

Abyssal Scepter increases all of your damage -- almost all of it is magical.

It stacks with thornmail return damage
It stacks with sunfire AoE damage
It stacks with Defensive Ball Curl Damage
It stacks with powerball damage
It stacks with Tremors damage
It even makes wriggle's procs and gunblade active do more damage.

Good build although i dont see reason for taking abyssal scepter it mostly benefits tremors, as main tank AP shouldnt be so iportant for you,rylais is better choise-some more hp+ap+nice aoe slow when using tremors,not sure but i think defensive ball adds slow efect too.