Psionics study the magic realm of Psi. They have the ability to manipulate anything related to the mind, whether that be causing instanity, curing insanity, detecting lies, hypnosis, etc. Because of the nature of mental spells, it's possible for a Psionic to kill an enemy without any apparent damage to them physically (damage to mental health). It's a nice alternative to hacking and slashing that giant Ogre standing in front of you. Of course, more intelligent beings are more resistant to mental attacks. Likewise, any entity that doesn't have a "mind" (i.e. that green slime in front of you) is completely immune to mental attacks as well.