Originally Posted by RaiTheSoul
You just have to participate in a Harkon defense group that finishes within 20 minutes or what?
Well, 20 minutes is the max limit, it actually ends when you defeat Chaos, which is around the 17 minute mark.
According the picture, you only have to participate in a group that wins, but I don't know. If you have an AoE, it's very easy to win the first Harkon Sanctuary.
Also, it doesn't appear that it's a reward that stacks. So it looks like a one-time-only reward.