Goto the room King Cronus is in, search and reveal the exit behind him. You'll need to run in/out of the room north until his mistress finally asks you to keep quiet about her. Use the social "nod" and you will be granted the mark.
For keeping King Cronus' dirty little secret, the Powers have granted thee a Mark. You have been granted the Mark of Secrets! You gain 261,000 experience points! You gain 48 practices!