Mental shield
A mental shield is a property that most creatures that lack physical brains have that makes them immune to mental damage and most mental effects. These are most often Robots. Telepathy also does not work on them.
The following effects cannot be applied to creatures with a mental shield:[1]
As esper hunters
If the player has a high enough glimmer and summons a psychic assassin, there is a chance for the hunter to be a creature that normally has a mental shield. The shield is removed, and thus is normally affected by mental attacks and effects.[2] Psychic thralls are unable to become psychic thralls.
References
This information is reliable as of patch 2.0.201.71.