In Lost Ark, one of the events that you can partake in is called Boss Rush. In it, you can pair up with three other players to take down bosses under a time limit. Before you can do this, however, you need to complete the Guide Announcement: Boss Rush Quest.
You can complete it at any time after having finished the main storyline quest (Ealyn's Gift Main Quest) and reached Level 50. You also need to have an Item Level of 802 or higher.
When do you unlock Guide Announcement: Boss Rush quest in Lost Ark?
In order to unlock the Guide Announcement: Boss Rush Quest, you must first complete the criteria above and then get your hands on a Boss Rush Entrance Ticket. You can obtain one of these Entrance Tickets as a random drop from both Chaos Dungeons and Guardian Raids.
Once you've done all of that, you'll have access to the Boss Rush events! You can use up to three tickets each time for a chance to earn more rewards.