Guild Wars 2

So, you need to jump up, dash into the room and kill all the inquest’s. I did that, but for some reason, my achievement was gone. Is there anything, that triggers the failure of it?

Is there a time limit on it?

You didn’t kill them quick enough most likely.

From the wiki:

Dashed Advantage

For this achievement, you need to access a hidden area during the final fight. Since you need sun aspects for it, you have to bring Aerin down to 50% of health so he activates his protection and sun aspects appear. Then, pick up an air aspect and jump up in the north-eastern area, where the sand is coming down. Follow-up the path, pick up the sun aspect and dash through the wall. There, a golem and a few more inquest are waiting. You have to kill the golem quickly before it reaches the safe area behind the protective wall, and then kill the rest of the inquest (not the Veteran Inquest Assassin though) to get the achievement.

My guess is that he didn’t kill the golem first and it escaped behind the wall. The golem should be your priority target; you can mop up the remaining Inquest easily enough afterwards.