In a recent game players were attack by a Glaberzu. It promptly tore them up and they ran.
Given three days down time, they were not able to find any information on the creature. They do not have Knowledge (Planes). Gather Information checks were not high enough to gain any information either.
DM maintains that it is cheesy that a player memorize Banishment as there is no way that player could know to banish it.
Player maintains that is illogical since the player (Cleric 12) can summon demons through the use of Summon Monster I - VI and can identify the spell used to summon the Glaberzu and can even summon a Glaberzu of his own with a scroll of Summon Monster IX.
Player further maintains that he does not need to know it is a Glaberzu, or a demon for that matter. Just that it can be summoned, and therefore that it can be Banished and that if he can summon it, he should clearly be able to know that it can be summoned.
Where do any and all RAW/RAI with regards to creature/spell knowledge (not just the knowledge skills but knowledge in general) and the ability to use spells fall into play in all of this?