Reflection is a property possessed by certain monsters and can be given to you by an armor of reflection. Reflective creatures have a chance to deflect all spells, both hostile and helpful, cast at them. There is an equal chance that the spell will be directed back at the original caster. Otherwise, the spell will be deflected in a random direction.
You can become reflective by wearing an armor of reflection. The chance to deflect a spell depends on the enchantment level of the armor.
An armor of immunity causes you to reflect all hostile spells from a specific group of monsters. In this case, the attack will always be reflected back at the original attacker.
Golems, stone guardians, guardian spirits, winged guardians, mirrored totems, and monsters with the reflective mutation are reflective. Golems and reflective mutants fuction as if they were wearing a +4 armor of reflection. This gives them a 48% chance to reflect spells and an equal 48% chance to send reflected spells back at the original caster. The others always reflect spells back at the caster.
The high chance of reflection makes these monsters undesirable targets for most staves. However, you can potentially use this to your advantage by targetting them with helpful staves in the hopes that the spell reflects and hits you. This is most effective with a guardian spirit as it is friendly and can be summoned with a guardian charm whenever you want.
Allowing an ally to become reflective has its advantages and disadvantages. The major advantage is that your ally becomes much more resistant to spells, including negation. The major disadvantage is that since the ally will reflect helpful spells there is a chance that any further attempts to empower them will fail. Another disadvantage is that if you have another ally that casts healing, protection, or haste, that ally will attempt to help your reflective ally but the spells may reflect and hit enemies instead.