The Flame of Hatred is a side quest in Novigrad. Its not an official side quest but you will earn experience.
Travel east of the Hierarch Square. On the street you will see a priest speaking ill of mages, alchemists, and especially witchers, as you will notice when you walk up to him. Speak to him to have a little conversation. Once the conversation is over the crowd will lose interest and walk away.Later while traveling around Novigrad, Geralt will get ambushed by two guards. They're trying to take revenge on you for insulting and humilating the priest. You have two options: Hand over your weapons which will result in Geralt fighting with his fists against their swords or you can not hand over you weapons and fight them with everything in your arsenal.