Fiend

Fiends are a type of creature, the two main types being demons and devils. They are amongst the creatures detected by the spell Detect Evil and Good and the Paladin class feature Divine Sense.

Demons and devils are in an eternal, ongoing war in the underworld, called the Blood War. The Erinyes found captured by Cansa Cinsmic in the Spire of Conjuration, Vastren, was a veteran of the Blood War.

Tieflings are humanoids with fiendish heritage. Vastren recognized this when meeting Mira, even though Suithrathus Luirmo wasn't familiar with a ‘tiefling’. Anrel uses this to explain to his friends what an Aasimar is, saying they're like opposites in terms of heritage. For this reason, tieflings show up as a dim glow to those using Divine Sense.