Gloom Prism vs Force Weapons

[Deathscythe91]

Can a necron spyder or tomb stalker use the 4+ nullification against the activation of force weapons?

[Nabterayl]

No. Gloom Prism specifies that the enemy unit must attempt to target a friendly unit with the psychic power, and force weapon activation doesn't target a friendly unit. The force weapon itself does, but the psychic test to add Instant Death to the weapon only targets the user.

[Patrick Boyle]

To be somewhat more clear just in case, the force weapon psychic power doesn't itself cause the target to suffer instant death, it's a buff on the psyker that grants Instant Death to the wounds they inflicted.