If it is a case of spell damage being increased - this could be done with nwnx
I released a modification to the nwnx_cool plugin.
One of the features is a Damage hook.
It allows you to determine the damage being inflicted to players or monsters, and actually modify the damage before it gets applied / displayed in combat log.
Using this logic, you could check for the focus item on each damage application, and if it is present, increase its damage by 50% etc
I use something similar except I hook it into custom feats.
For instance : If a player of mine has a feat called 'Adaptive Flesh'
Their skin actually adapts to provide immunity to damage types over time.
If they suffer 18 consecutive attacks of fire damage, they gain up to 90% immunity to fire. However - their Skin can only retain memory of one damage type.
As they gain immunity to one damage type, it decreases any immunities they may have already by 5%.
However I have it setup that they get a 5% base immunity to all damage types by default.
This is done through the damage hook.
What your doing is kinda in reverse - but still do-able.
Instead of me checking to see if the player has a feat
You need to check that the person who damaged the monster, has an item.
The hook provides access to DamagedObject + DamageInflictor - allowing you to get the data needed to create your focus system.
http://www.nwnx.org/...5d7959d3559d972
Source code and compiled binary link can be found here.