Psi bolt is the internal name for a monster spell (though it is never referred to in the game as such). This spell can be cast by any monster that uses mage spells.
This spell will deal (monster's level/2)d6 damage, plus additional damage based on the monster that cast the spell: +8d6 for Orcus and Demogorgon, +6d6 for Dispater, +4d6 for Archons and golden naga, +2d6 for ki-rin and +1d6 for other monsters. Neferet the Green is an exception: her Psi bolt spell does (her level/2 + 2)d8 damage. 
Because of the way monsters select their spells, the relative frequency of this spell decreases as the monster's level increases from 1 to 23, but the relative frequency increases as the monster's level increases from 23 to 50. Thus, spellcasting monsters with a very high level (such as Demogorgon) will use this spell very often, which can be dangerous to the hero, as they can do a lot of damage with this spell (for example, 33d6 base damage for Demogorgon).
|"You get a slight <head>ache."||0-5|
|"Your brain is on fire!" (not to be confused with intelligence drain)||6-10|
|"Your <head> suddenly aches painfully!"||11-20|
|"Your <head> suddenly aches very painfully!"||21+|
In SLASH'EM, there are many new spellcasting monsters. For the ones that can cast mage spells, the additional damages values (as explained above) are the following: +6d6 for ghoul mages, +5d6 for Solars, +4d6 for Planetars and ghoul queens, +2d6 for vampire mages, and +1d6 for all others.