A name may seem an innocuous thing, but like story and faith, it holds limitless symbolic and sympathetic power. The true name of a being is a direct link to their soul.
Discovering and speaking the true name of a monster can be used to bind, banish, or gain power over them. However, just like all secret knowledge, it is dangerous for the incautious.
Using the true name of something evil can draw its attention upon the speaker. If they are not suficiently prepared and strong of body and spirit, the monster will be the one in control.
It can be dangerous to speak the proper name of a type of monster, lest one be listening. This explains the common use of nicknames for the fae, fiends, and even death.