Occultist's Mind Barrier (Sp) is an ablative barrier to taking 'points of damage' which one would assume would be hit point damage given the descriptive text.
As usual, if a target suffers no damage from an attack the 'rider' effect generally relies on doing damage thus it is not triggered and the target is not affected.
A good mundane example would be a poisoned dagger with an injury poison.
It can get trickier with magic as the secondary effect usually relies on the target taking damage BUT it is not universal.
in your example: Wraith LE Medium undead (incorporeal) CR5, and incorporeal adds a twist.
> Melee incorporeal touch +6 (1d6 negative energy plus 1d6 Con drain)
> Constitution Drain (Su) Creatures hit by a wraith’s touch attack must succeed on a DC 17 Fortitude save or take 1d6 points of Constitution drain. On each successful attack, the wraith gains 5 temporary hit points. The save DC is Charisma-based.
thus it is not a 'rider' as the wraith need only touch the target to inflict BOTH. This is also supported by the Dread Wraith description which follows the Wraith description. I believe there's been some RAW discussion about Wraith doing both with one touch but I'm going with the simple interpretation given the monster's description.
The Mind Barrier will adsorb the HP damage from negative energy but is of no use against the Con drain.
The twist is that incorporeal requires a [force]/Ghost Touch for corporeal effects.
On wording: The lack of the term 'hit point' from the damage does make it a bit nebulous and 'harm' is very generic. The examples given make it clear it is regular HP damage. They could have given a more open example like a Vampire's touch but they did not. IF it was Con "damage" (Abil Drain & Dmg) there might have been interpretive squiggle room based on pedantic terms.
