Then modify it a bit.
Treat the focus as having a number of 'health' boxes equal to its Force.
Insert preferred contested roll between enemy mage and the focus with the pseudo-damage being applied to this 'health'.
When foci 'health' drops to 0, the enemy mage has effectively 'hacked' the focus, overriding its aura/energy/pattern and forging a temporary link with the result that the focus is no longer applying its bonuses at this time for the wearer but is still magically active.
Attacking mage can now opt to the following:
A) Shuts it down and breaks the link
B) sustain the link holding the item in limbo (no bonus from the foci for the target , -2 dice penalty on all actions as per normal sustaining for the enemy mage) and do other actions as normal
C) sustain the link as per B and do an old fashioned grounding spell through it. As the foci serves as a link to the owner, this reduces the dice pool of the defending mage by a certain amount (maybe equal to half the force of the foci, rounded up) against spells cast through the foci in this manner. AoE spells can also be shoved through with the owner/foci as ground zero, but only the owner caster suffers the dice pool penalty, others around him get normal resists though they may wonder why spells shooting out of the mage and hitting them.
Casting enemy mage still has sustaining penalty while doing this.
D) Destruct (Raises the possibility of destroying it, but no idea on how to balance a system for this yet)
If option A is taken the foci is down for 1 combat Turn before it can be reactivated, a reset so to speak. Foci Health now back to full and back under owner control.
If option B or C is taken the owner of the now compromised foci can attempt to regain control with a simple action (once per action phase-counts as attack action). Insert plausible contested roll method between the two mages here.
If enemy wins link remains up with the foci still locked up , owner can try again next action phase.
If owner wins then he has broken the link, however foci is disabled with it's Health at 0 but will regain 1 'health' per combat turn with bonuses returning once back to full health. Owner would be wise to shut it off to reset otherwise someone could attack the foci and only needing to bring it back to zero ( or 1 hit if already at 0) to compromise it again. If shut down by owner, must wait 1 combat turn to be able to reactivate with Foci back to full health. So resetting is better solution and faster.
OR If the owner mage does not want to risk a battle for control, he can automatically break the link by physically removing the foci from his person/personal space with a simple action (as per picking up/putting down an object), ie ripping it off and dropping it so it is no longer in contact with owner mage. Shut down occurs and link is broken. Foci can be picked back up with a simple action but still needs 1 combat turn from when it shut off to be reset before reactivation is possible.
No plan for option D yet, have to find some ruleset that could make this possible without being too easy or impossible. Item loss, especially when karma is paid for it, is a very sensitive issue and should not be lightly done.
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The idea is very rough and would need tons of polishing, but will leave it to the megamages to come up with a better version.