You need to be Level 30 and above to do this party quest.
For a complete video guide of this PQ, click the Play button above (thanks to AionJC).
Either head to Ludibrium: Eos Tower 101st Floor <Exit Map> or enter the map through the Dimensional Portal (shown above), and form a party of 3 or greater up to a maximum of 6 people. The recommended levels for this quest are 20-69.
Have your party leader click on Red Sign. You will have 20 minutes to complete this party quest.
Note: Having a Cleric in your party is highly recommended for the boss fight. All-Cure potions are also highly recommended, as the battle with Alishar will occasionally leave your skills locked or your character being inflicted with the darkness effect.
(This quest is also known as the Ludibrium Party Quest.)