This is unlocked once you have completed 10 major arkfalls. A major arkfall appears as a large red circle on the map with small red circles and arkfall symbols scattered around it. To finish a major arkfall, all the minor ones within the large red circle must be completed. After this, a final arkfall symbol will appear with a boss fight. These are tougher than minor arkfalls, but dozens of people typically show up. Again, just make sure you contribute some damage during the event and your achievement will pop at the score screen.