One of the players of the oneshot campaign that I’m writing chose the feat Pierce Magical Concealment (Complete Arcane, p.81), as a DM I’m not sure how it would work in an encounter I am planning.
The manual states about the Pierce Magical Concealment feat:
You ignore the miss chance provided by certain magical effects.
Your fierce contempt for magic allows you to disregard the miss chance granted by spells or spell-like abilities such as darkness, blur, invisibility, obscuring mist, ghostform (see page 109), and spells when used to create concealment effects (such as a wizard using permanent image to fill a corridor with illusory fire and smoke). In addition, when facing a creature protected by mirror image, you can immediately pick out the real creature from its figments. Your ability to ignore the miss chance granted by magical concealment doesn’t grant you any ability to ignore nonmagical concealment (so you would still have a 20% miss chance against an invisible creature hiding in fog, for example).
This doesn’t specify if a character with this feat can actually see an enemy that is, for example, under the effect of the spell Invisibility (Player’s Handbook, p. 245).
I am planning an encounter in which the party will be invited by a wizard for dinner, an assassin under the effect of invisibility will pretend to be an Unseen Servant (Player’s Handbook, p. 297) until the wizard gives him the signal to attack the party.
Will the character that has the Pierce Magical Concealment feat be able to see the assassin, or at least have any advantage in noticing he is not an Unseen Servant?