This quest is only for Blade Knight, Muse Elf and Soul Master. Other characters such as Magic Gladiator or Dark Lord are not required to do this quest.
How to find Marlon
Go to Lorencia, Noria, Devias or Altans. He hides in one of these cities.
Start Your Quest
Once you found Marlon, talk to him.
He will ask you to bring him Ring of Honor which can be bought from NPC in Lorencia's Bar or found in Tarkan.
Return with Ring of Honor to receive your next quest.
After completed the first part of the Marlon's Quest, he will then ask you to search for Dark Stone (Blade Knight) as the second and last part of the quest. Dark Stone can be bought from NPC in Lorencia's Bar also.