This omen requires you to replace all faceup adventure cards in a region with Harbinger cards. I popped this in a heavly populated Dungeon, replacing dozens of faceup cards. While doing this, I revelealed four or five events. Should I leave them in the spaces, to be activated when someone lands on them, or trigger (gulp) all of them immediately? I think leave them until landed on, but am not sure.

Thanks for your thoughts.

It happens only once in a while, but it's possible for Events to remain faceup on the board. A typical example from the base game is drawing the Imp on the Hidden Valley, along with another Event, but if you encounter the Imp first you are teleported and the Event remains faceup.

Cards are resolved only when encountered, so in this case all Events remain faceup waiting to be encountered.

Thanks Warlock. That's what I thought too, I just don't recall seeing it before.