Alright, to specify a bit more:
Requirement 1: You need to have captured Raeve keep, obviously. At this point you can either claim Helayna as your own or leave her to her fate, it doesn't matter.
Requirement 2: Sometime around week 30 (I don't remember exactly), Helayna will ask for your help in escaping. At this point, you can either tell her it's too dangerous and refuse, or agree. If you claimed her, you can also convince her to stay by telling her you love her or something along those lines. In order to progress to the dungeon event, you have to refuse to help her escape AND not convince her to stay.
Requirement 3: Sometime around week 35 (I don't remember exactly), someone will help her escape. You'll have to go through several choices to track down the culprit, and if you do so successfully and turn the culprit in to the twins, the dungeon event will start the next week and take four weeks to complete. You can also get it to trigger by offering to punish the culprit yourself, taking it easy on them, and failing a rolled check to see if you were convincing; though obviously leaving the culprit to the twins is the easier route.