I initially chose this as the plot for my overlord in my recent campaign start, but got confused with the zero-cost "key is found" "Keys are lost" cards...when are they used? the explanation doesn't appear very clear nor does the naming - as with the Leviathan plot, which I wound up choosing. The Leviathian plot has all of them named in order, and has conditions to be met for each one.

Chern

The one that gets put into play at the start of the campaign is "An Ancient Weapon", as the card text describes: "Place this card in play at the start of the campaign [...]".

The way I interpret the two others (and I'm fairly certain it's the correct way to interpret them):

The card "The Keys Appear" can be played during an Overlord turn, and really starts off the hunt for the keys; before that, the heroes know there will be three keys to find, but nobody knows where they are. So the Overlord basically decides when the heroes have to start competing with him in the hunt for the keys.

The card "A Key is Lost" finally, it makes only sense to play that one when a key is actually in a vault and thus considered "safe" by the heroes. It bails out the key and reopens the chase for that one. Essentially, it's a bail-out-of-jail card for the Overlord, to deny the heroes the possibility to foil the plans of the Overlord too easily merely by locking away one key in a well-fortified position. The fact that it costs 0 to play is merely a coincidence (or rather, a balancing decision to make it actually a good choice to play for the Overlord when the situation arises where it could be useful).

"A Key is Lost" is to create a balance fix when the heroes left a keystone on the island map or Dallak where none of the overlord LT can reach it.