You need to be between Level 44 to 55 (inclusive) to do this party quest, which is also known as the "Forest of Poison Fog Party Quest" and "Ellin Forest Party Quest".
For a complete video guide of this PQ, click the Play button above (thanks to AionJC).
In order to enter this party quest, it is necessary for the party to be consisted of at least 4 members, which may be of any Adventurer class. Upon entering, the party will be transformed to look like the Legendary heroes, based on their job.
Because this party quest can only be attempted by Adventurers, the Knights of Cygnus will not be able to participate.
Once the party leader has created a party with at least 4 members, talk to [node:8018 link] who is located in Ellin Forest. You have 30 minutes to complete this party quest.