Floating balls of pure energy and anti-energy, that is what we are talking about here. The idea is definitely more sci-fi-fantasy than traditional fantasy.
The 1e MM2 suggests they would be used mainly as guardian monsters, summoned to deal with theft.
They would also be cool if used as a "bodyguard" monster of some wizard, tagging along with him, operating at his beck and call.
Also, they would make a very spooky and intimidating "assassination" monster, sent by a wizard to kill his enemies.
Another nice plot hook would be to have one of these locked into a cube of force in some mad wizard's lab. Of course, it would escape, mess up the wizard in the process, and take off to float aimlessly through the countryside (or underground)...
However you use them, they would be unusual and mysterious. They have no natural ecology, and they are "mindless by human standards", meaning some sort of alien intelligence, nothing in common with us at all in terms of motivation or empathy.
attacks: tendril slap or energy blast (7-12 damage)
Magic Resistance: 15% (+1 or better to hit)
attack/defence modes: all/all
Psionic powers: none listed, but it would make sense (and make them more bad ass) to equip them with Energy Control and Telekinesis.