For the most part, a magical artifact is a tool or object with supernatural powers, and is detectable as arcana. Very rarely, an artifact may possess a will of its own. Most are subtle, with psyches that can't really be called minds, and intelligences that aren't quite human; any direction or communication will seem like intuition. A few have such clear personalities that they can carry on extended conversations with their bearers. All artifacts with will use the same system.
Such an artifact is rated with an Intelligence score, a Psyche score, and a Will score. Will is Int + Psych, modified by the artifact's magical powers. A bearer's Will score is Cha + Wis, modified by Str. The difference between these Wills is called the Control Factor, or CF. A positive CF indicates that the bearer is normally in control, but this control may be tested from time to time - particularly when the bearer has been injured or weakened. A negative CF means the bearer is normally allowed free reign, but the artifact can and will seize control whenever it desires.
Control is tested when the bearer first picks up or attempts to use the artifact. It will be tested again when the artifact has an opportunity to pass to another, more desirable bearer, or when the bearer attempts to discard or replace the artifact. If the artifact has a special purpose or motive, it will test control whenever it has chance to pursue its ends. It will also test control when the bearer first falls below maximum HP, and again when the bearer falls below 50% maximum.
When a test of wills is indicated, dice will be rolled: 1d6 if the bearer is at less than maximum hp, or 2d6 if the bearer is at less than half maximum. If the bearer is the wrong alignment, roll an additional 1d6+1. Thus, if the bearer is of the proper alignment and unharmed, then as long as the CF is positive, the artifact is unable to exert any control. If the CF is negative, the bearer will automatically fail any test of wills.
Any artifact with will has an optimal alignment. This alignment may or may not reflect the artifact's own purpose; it is simply the alignment best suited to maintaining control over the artifact. Sometimes there is a true arcane compatibility at issue, but in other cases it's simply the fact that a mind that works a certain way can best curtail a given artifact's attempts at dominance. Wrestling with the artifact's will can be exhausting; a would-be bearer of the wrong alignment will lose 1d6 or 2d6 HP to exhaustion, depending on how much his alignment differs from the optimum. (This loss of HP will affect the test of wills, making it very difficult to successfully master a hostile artifact.)