I was thinking of using Will and the Will Static Maneuver table, but I was just trying to think of how to generate the number the character needs to beat. I was thinking the RR table might do the trick. Actually maybe I should check the Treasure Companion book and see what the average level is of some of the legendary artifacts there. It might help be generate a number.
Thanks!
edit: In looking at the table I noticed all artifacts except for one are level 75 or higher. This would mean if I used the RR as a base to beat, the player if at level 5 would have to get higher than a 135. I have noticed the average will power for a person is going to be between 40 and 80. The lower end for strict fighters and the higher end for more empathic characters. At a level 10 they would have to beat 120.
It's hard to gauge just how powerful these devices could be. One would have to guess. My guess is they would be quite powerful given that Sith are quite capable of control and manipulation. Your average Jedi would have a higher will power to withstand it but even a Jedi shouldn't be given special treatment I would think. Anything higher than a 75 I think is just pushing it. I mean, If I made the Holocron a level 100, that would mean you would have to beat a 160. Even for the best that's a roll higher than an 80. I would think there should be some balance to the artifact.
In reflection I think a will power roll using the RR table as a reference for the number to beat taking the result and applying it to the Will Contest table is a good way to go as you described. I'm also thinking a good level for the Holocron is 75. It should be VERY powerful, but still beatable by characters with a strong will, 80 or better.