Possible Quest Bug Or I'm Doing Something Wrong

The next patch will change the quest log to clarify what you need to do in this case.

Which is, you to get the Apex by either killing all the bandit lords, or completing one of the bandit lord quests.

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babuzi wrote:

Ur buddys startet the encounter before u had the quest, so it does not count to u. U have to kill him again now .

Actually, it doesn't matter who started it. It does matter who is present when the boss dies.