I read the series when I was younger. It's about a young man who is forced to leave his sect behind and journey out into the city. He is described as with a pale complexion due to his whole life living below ground. His sect worships a god of death, and as such confers natural advantages such as immunity to poison. This comes in handy for the boy as he accidently insults a noble and is challenged to a duel, not knowing that the noble used poison on his weapon. The duel ends when the Noble becomes scared at the end of duel wondering why the boy hasn't died yet from the poisoned slash on the boy.
This is The Shadow of the Torturer, book one of the Book of the New Sun series by Gene Wolfe.
The hero Severian is a trained executioner and is a member of a monk like order called the Torturer's Guild. He is sent out for reasons that only become apparent later in the series.
The noble isn't really a noble, but a fraudster called Agilus trying to get Severian's (extremely valuable) executioner's sword. He masquerades as a hipparch of the Septentrion Guard and challenges Severian to a duel using as weapons a branch from a poisonous plant called an avern. Severian is hit by the branch and collapses, but survives due to immunity, gained from his training in the torturers guild. When Agilus sees this, he runs away in terror and forfeits the duel.