Where do they go? There are two schools of thoughts in this area. Some believe the banished creature is cast into oblivion - dies. Other more sensible thinkers believe they simple return from whence they came.
Banish is a third-level order magic spell that costs five spell points and destroys a number of summoned creatures based on the caster's skill.
When facing off against an enemy Druid or Demonologist, or even a stack of devils, the ability to destroy their summoned creatures can be the difference between victory and defeat. It also destroys Illusions and Phantom Images. Note however that this spell is negated by the Ring of Permanency. Dispel is much more useful in destroying illusionary stacks, however, as it will destroy the whole stack at once.