Your character has advanced or scholarly knowledge of an exceedingly obscure, bizarre, or otherwise eclectic subject. When you take this feat, your character gains proficiency with a skill of your choice that is appropriate to the subject. For example, a character with advanced knowledge of demonology should gain proficiency in arcana, or religion. The PC should work with the DM to determine what exactly defines their expertise. It should be appropriately narrow (say a specific time period of history, school of magic, or type of monster). When a pc with this feat investigates something that is directly related to their field, they roll with expertise.  

The PC has has a strong chance of knowing (or at least knowing of) other experts in their field and adjacent fields. There is a 65% chance they can identify a reliable expert and/ or work on the subject, a 25% chance they are familiar with the expert, and a 10% chance they know the expert personally or have intimate knowledge of the text. See chart below:

1-35% no knowledge, 36-65% can identify name or work, 66-90% familiar, 91-00 intimate knowledge. 

The PC may be capable of teaching the fundamentals of the subject to others and, given enough time, can bestow proficiency in a closely related skill with an appropriate teaching period. It is up to the referee how long the time period should be, but it is recommended that it not be any less than 3 months if a character is devoting an average of one hour a day to studying the skill.