Thunderlord's Decree (TD) deals almost the same base damage (10 less per level) as CotC grants shield strength at a slightly faster cooldown (scaling to twice as fast), so I think the bases are fine.
However TD only scales with 30% bonus AD and 10% AP.
Versus CotC scaling with 7% MAX HP.
I think the scaling is pretty broken. Right off the bat, for the entire game the base HP of a champion will give CotC a bigger shield than TD does damage with any build. E.g. a late game fighter like Riven would have 2020 base HP at level 18 and with a build of 
she'd have 250 bonus AD.
7% of 2020 is 141.4
30% of 250 = 75
That's a pretty broken disparity alone. Then you need to consider that TD requires 3 stacks to proc (abilities or autos) while CotC requires only 1 hard CC and then gives that big of a shield for EACH enemy nearby (not even each enemy hit!!).
That's completely out of line. On a champion that with full build that has 0 health and 4 strong AD items the tank mastery provides a shield that is nearly 2x as big in the worst case (1 champion) and scales to over 9.4x bigger in the best case (5 enemies) from a single ability hitting any enemy compared to a mastery that scales with the stats built off of 3 damaging procs on the same enemy (considering scaling components only as the bases are similar). Makes absolutely no sense whatsoever.
We know that buffing TD would be broken. Damage dealers don't need that much more damage to kill squishies. Fighters however don't need that much more tankiness. It'd be one thing if it was a Shen or something coming in with this shield, but a Riven or Poppy or something getting in there and being tanky as hell despite dealing real damage is absurd.
CotC should have its scaling changed to 10% BONUS HP it would be more reasonable.
Already, health is more than 12x cheaper to build than AD (so stacking health versus stacking AD, you're getting the scaling 1/3 * 12 = 4 times easier). It's reasonable to give it a bigger scaling since offensive champions are a lot more likely to heavily stack AD compared to a tank outright stacking health (they'll build resistances and various passives/actives that they need as well). This would prevent any fighters who only build 1 or no health items from gaining such a huge benefit from it simply because they have a stun in their kit though... Meanwhile it'd also tone it down to reasonable levels on tanks (which, who are we kidding, you're probably better off just waiting out the shield on them).
who can't utilize it freely without building bruiser as opposed to melee carry.
