My cleric is trying to combat undead creatures. I wanted to cast the celestial brilliance spell from Book of Exalted Deeds.
However the descriptor of that spell is [Good] and [Light], while my cleric is Evil. Can he cast the spell? Where in the rules is this explained?
I clearly do not understand the purpose of those descriptors.
I found Can a Neutral Good cleric cast Lawful or Chaotic spells?, but I do not know where the answer is coming from. What book explains this?
Best Answer
A cleric cannot cast spells with an opposed alignment descriptor
Taken from the Cleric description in 3.5 SRD - similar rule appears in PHB:
So as an evil cleric, or a cleric of an evil deity, your character is forbidden from using any [Good] spells.
I'm not aware of any rule prohibiting an evil cleric to cast spells with the [Light] descriptor.
Regarding spell descriptors...
I recommend you read the SRD text about spell descriptors. It isn't very long, and says (among other things):
For example, spells with the [Good] descriptor have special effect on certain evil outsiders, can't be cast be evil clerics, and can be detected using the Detect Evil spell. Similarly, some creatures (mostly undead and darkness-themed) are more vulnerable to [Light] spells.