A simple scenario: a psyker uses Compel on someone holding a Hellblade and orders the weapon be put away. Assuming the psyker wins the contested roll, does the target still have to test WP to put down the weapon, as per Hellblade special rule? Or does the enchantment of the blade take precedence over psychic powers, granting the warrior a sort of "second save" against this particular brand of mind control?

What do you think?

Personally because the Psyker is the one making the blade get put away I would make the psycher make the roll.

Just like someone will still need to roll a medicae check if they are compelled to heal someone, so too will the target need to make a willpower test to put away the Hellblade (him and no one else, the psyker doesn't completely supplant the target's mind, just forces them to do something)

I rule that the players have to declare what tey want to achieve with Compel before they roll the opposed willpower test, and modify the difficulty of the test based ont hat. I'd give the psyker an additional -30 modifier along with any other modifier that might apply if he wanted to force someone to put down a hellblade.

That's something to consider. Thanks for the feedback, and please keep it coming!