You have to decide if they are to be treated as a divine power/tenser effect (same as the spells, like azz3 cloak or UR plate), or added attacks like Clockwork Boots and Harper Boots (that do not stack with added attacks from class feature like CoT/SD afaik).
Assuming they're paragon loot, what build should they complement? ''Moar damage for everybody'' would cause too many problems I'm afraid, and the base item is quite good, compared to the ''meh'' Wrap of the Dark Prince that has a lot of cosmetic stuff but gets used only for the active power pretty much.
Non dispellable, stacking bonus attack on a 10% phis immune amulet? OP! OP!
Make it cast a (dispellable, about 5 minutes lasting) Divine Power at CL 50 and call it a PL alternative to the aformentioned items on a different slot to solve stacking issues and class balance (it is still damn powerful, but nothing new)
Well it does costs precious ac for Str tanks - maybe 5% phy imm instead?
I would recommend giving it at least 2 charges - It is known that tanks will die when a nasty roll 1 occurs. (I would give it 3 charges). It can also become character bound to offset this uses per day increase.
This looks like it was taken from your Adamant Justice suggestion, which was already tweaked and slated as set loot for Limbo. If you still want one for this slot, we can take a look at this down the road.
We implemented as nonstacking with toyshop, must be worn.