High-level vocabulary/archaic:
* Bellicose: (adj.) Demonstrating aggression and willingness to fight.
* Beneficent: (adj.) Doing or producing good; kind and generous.
* Blasphemous: (adj.) Sacrilegious against God or sacred things.
* Bombastic: (adj.) High-sounding but with little meaning; inflated.
* Bona fide: (adj.) Genuine; real. (While known, its Latin origin can make it challenging for some)
Technical/Specialized:
* Byzantine: (adj.) Extremely intricate and complicated, in a way that is hard to understand. (Often used metaphorically)
* Biogenesis: (n.) The origination of living organisms from nonliving matter.
* Biometrics: (n.) The measurement and analysis of unique physical or behavioral characteristics, often for authentication.
Words with nuanced meaning:
* Beguile: (v.) Charm or enchant (someone), sometimes in a deceptive way.
* Belie: (v.) To give a false impression of.
* Broach: (v.) To raise (a sensitive or difficult subject) for discussion.
This list is not exhaustive, and the difficulty of these words will vary depending on the context and the individual. Some words may be easily understood in certain contexts but challenging in others.