Quote Originally Posted by masteraleph View Post
Only sort of. It requires Weapon attacks, which is a bit of a drag for a number of characters, particularly actual Assassins.
Mordant and Spiderkissed Weapons use the exact same language as Sunblades for their damage modification:

"All damage dealt by this weapon is XXX damage. Another free action returns the damage to normal."

If you're capable of using the weapon as an implement (swordmage multiclass), even your implement attacks would be able to do poison damage since you're still using that weapon (just as an implement).

Also, point of interest about the Mordant Weapon: it doesn't just turn it into poison damage; it turns it into poison and acid damage, making it much more difficult to resist (I can't think of any creature off of the top of my head that resists both of those damage types simultaneously, which would make it effectively unresisted). I'm not entirely sure how that would interact with immunity though, since I could only find the rules for combined damage types interacting with resistance. My gut instinct would be to act as if immunity is simply infinite resistance such that only a creature immune to both acid and poison damage is immune to combined acid and poison damage, but that might not be the case.