The Wand of Negation is pretty straight forward. It cancels all immunities granted to the opposing champion, even if that immunity comes from a source other than the opposing champion or his attachments. In other words, a pool with the Codex of the Infinite Planes [4th-452] in it would not grant immunity to a Greyhawk champion that was opposing a champion holding the Wand of Negation.

Because the Wand specifically mentions that it negates immunity to offensive magic items, the order of activation for this item is skewed. Simply consider it's effects first, before all other cards. Because this card causes no hindrances, this out-of-order activation will not unbalance a combat. The wand even overpowers opposing attachments which grant immunity, such as the Orb of Power [4th-170].

This wand has no effect on cards that hinder the play of certain cards without a stated immunity; i.e. Flesh Golem [4th-235] would not be affected by this wand, and the Golem would in turn discard the wand, because the Flesh Golem does not grant immunity to magic items, just removes them.  The Wand of Negation functions against champions with immunity to magical items, and against champions with the Orb of Power.