Netherworld Knowledge

With this proficiency, a character learns about the cosmology and organization of the game multiverse, focusing primarily on the ultimate destination of spirits after death: the Outer Planes. In addition, the character learns about the behavior of the dangerous creatures that inhabit the nether regions, including such fiends as the tanar'ri and the baatezu. As with necrology (which applies exclusively to undead), netherworld knowledge can reveal the specific weaknesses and natural immunities of beings from the Outer Planes. The proficiency can also be used to classify the exact type of extraplanar creature encountered. Both of these abilities require an ability check, however.

[Complete Book of Necromancers]