"Within Wynnadia's Corzine casino, the only one who has never lost a game is Triumph. As long as it appears, you are all but assured of victory, whereas unbelieving gamblers are sure to lead their families to ruin."
The spawn for Triumph or the other rare is usually triggered after fighting mobs on your way through the beginning of the dungeon. If the mercenaries do not appear talking on the rug at the spawn point, jumping on all the tables inside the dungeon will help to make them and the dragon appear.
Some people say the best way to spawn Triumph in Corzine is by hitting the 4-5 blonde girls till they start to run away from you. It is important you don't kill them but just to make them run away or Triumph (or Stoic Stitchscale Dragon instead of him) might not spawn.