Well you could always copy the ability of an orb that grants you the ability to attack air units, and make it bonus damage equal to 0 (I recommend orb of fire to have only damage effects without poison effects or so).
You have 2 solutions (that I can come up with):
1) Add that ability to the abilities of your mask of death
2) Disable the item ability data, and make it a unit and/or hero ability.
Then, with a simple trigger calling the event "A unit acquires an item", with the condition "Item being manipulated equal to mask of Death", add that new custom ability to the triggering unit, ET VOILA.
Of course, the side effect of this trigger is that it would be an ability given to the unit permanently, so other triggers would be needed to remove that ability if the unit loses its copy of the mask of death.
I would recommend solution number 1, because there would be no need for any trigger.