This is an attempt to establish a universal rule for mental "conflict", replacing the "Artifact Control Check" rule applied to artifacts and the "Battle of Wills" rule used for ethereal possession. As I was writing this, it almost turned into a complete revision of Psionics... but I resisted the temptation, and it is still consistent with Harnmaster (more or less). Let me know how it works and what you all think of it.
Mental Conflict or Battle of Wills occurs when two Wills/EGOs are linked/fused, and either or both of them attempt to achieve dominance. This might occur, when a character tries to invoke a special (EGO-dependent) power of an artifact (Artifact Control Check), or when an ethereal attempts to possess a living being.
Example: Bralon, a human with a Will of 14, has recently acquired a sword called Oathbreaker with an EGO of 12 and the power to inflict 3 additional points of Edge Impact. Oathbreaker has no Morality and no Purpose. In order to employ the power, Bralon must initiate Mental Conflict. Bralon's basic MCF is 14x4=56; Oathbreaker's is 12x3=36. Bralon has used the sword previously so the GM awards him a bonus of 0.5xWill=7. Hence, Bralon's MCF is 56+7=63. Oathbreaker has a Ground advantage (+1xWill); its MCF is 36+12=48.
Example: Bralon achieves MS and Oathbreaker rolls MF; Consequently, Bralon controls his sword's powers and can use them to further discomfit his opponent (Mong the Destroyer). If Oathbreaker had rolled CS instead of MF, it would have won and would likely have acted (subtly) against Bralon. This might cause Oathbreaker to inflict 3 points less impact than normal, but the sword might choose some other way to hinder Bralon, such as reducing his EML by 5-10 points who knows what evil lurks in the "hearts" of swords? Especially swords with names like this one...
If both sides achieve the same success, the battle is tied, neither side wins a dominant position, and the situation (whatever it may have been) is unchanged. If, after a draw, both parties (or the only live entity) in the conflict want a rematch, it may be resolved immediately.
Example: Bralon and Oathbreaker each achieve MF a draw. Bralon is fighting a melee and really needs Oathbreaker's powers because he is having difficulty penetrating his Mong's armour, so he initiates another battle of wills; he can do this unilaterally because he is the only live entity in the conflict. Again, it is a draw, and Bralon forces another battle; this time he wins and may employ the sword's powers.
Note: Mental Conflicts, especially those with Artifacts such as Bralon's stubborn sword, do not require as much concentration as would the casting of a spell, or the performance of a ritual. It is possible for Bralon to invoke Oathbreaker's powers while he is actually in the middle of his fight with Mong the Destroyer. However, since Mental Conflict generates fatigue, Bralon runs the risk of tiring himself out, and this could render control of Oathbreaker's powers somewhat moot (and Bralon somewhat meat).
Example: The average of Bralon's Will (14) and Oathbreaker's EGO (12) is 13, less the difference (14-12=2) is 11. Hence, a conflict between them will take 11 seconds.
Fatigue Points acquired from Mental Conflict are the same as any other Fatigue Points and should be recorded and applied (as part of the Physical Penalty) in subsequent Mental Conflicts and all other physical activities.
An artifact or dominated entity, or any entity without an Endurance Attribute recovers from fatigue at a rate of 1 FP per minute.
Any fatigue accrued by a controlled artifact or entity is passed on to the controller; FP acquired before it was dominated are not passed on.
After a suitable lapse, the dominated entity may initiate another Mental Conflict to break the dominance. The normal minimum lapse is equal to ten minutes x dominator's Will, but some artifacts/entities will wait until the dominator is fatigued or injured, or off guard to make their moves. This depends on the suppressed entity's Intelligence or cunning (if any).
In other words, Bralon would only have to win control over Oathbreaker once, at the beginning of his battle with Mong the Destroyer, provided Mong did not disarm him, and as long as the battle lasted less than 140 minutes.