Special; Resisted by Will 
Sends an extradimensional visitor (e.g., a demon) back to its plane of origin. 
It can only be cast by a caster in his home dimension. In an alien plane, you could not “banish” yourself back home, but a native of that plane could banish you. This spell will not work on a creature that is already in its home dimension. 
Resolve the Banish attempt as a Quick Contest: the caster’s Banish skill vs. the subject’s Will. If the caster wins, the subject immediately returns to its home plane. It cannot return for one month. Anything that it brought with it when it appeared (e.g., weapons) will vanish with it. Other things it may be carrying (e.g., screaming victims) stay behind. 
Note that certain powerful creatures are resistant or even immune to this spell.

Modifiers: +4 if the caster knows the entity’s “true name”; -5 if the caster does not know the subject’s plane of origin . . . and an extra -1 if he believes he knows where the creature came from, but is wrong!

Cost: 1 point per 10 character points the subject is worth. Minimum energy cost is 10. The caster will not know in advance how much energy the spell will require, and may fall unconscious or even wound himself in casting the Banish. 
Time to cast: 5 seconds. 
Prerequisites: Magery 1 and at least one spell from each of 10 different colleges.

Item 
Some “holy” artifacts are of aid in banishing actual demons, and items from the home planes of extradimensional creatures may help in understanding and banishing them. Some rumored amulets worn by planar travelers help them resist banishment.