Reynald de Chatillon is a Fallen Paladin who became an outcast in the Order of the Radiant Heart after going against the vows he made to the Order.
In the Seek Out and Investigate the Fallen Paladins subquest, you will find Reynald and other Fallen Paladins fighting with smugglers. After the battle, he will ask you to retrieve Anarg's Cup from the Order of the Radiant Heart building in the Temple district. After Reynald recieves the cup, Anarg will appear and say your party are spies. You will then have to kill Reynald.
Reynald's name is probably a reference to the real-life crusader and Prince of Antioch Raynald of Châtillon