Does Dahan on their Guard *(In each land, 1 Defense per Dahan)* provide Defense based on the number of Dahan at the time the Fear card was revealed or when the Ravage happens?

Does Dahan on their Guard (In each land, 1 Defense per Dahan) provide Defense based on the number of Dahan at the time the Fear card was revealed or when the Ravage happens?

For island-wide effects like Dahan on their Guard, you check the number of Dahan there when the Invaders do damage (usually in the Ravage).
This applies to all Event / Fear cards that affect all lands with (some contents) - you treat it as a game-wide rule for the turn, so you don't have to keep track of where pieces used to be across the entire board. Powers which affect all lands of a particular type work the same way, for the same reason.
Powers which affect only one land check land contents at the moment of use, again to minimize required memory: it's easier to remember "this specific land has Defend 3" than "this land has Defend equal to the number of Dahan in it at the time".
Unusual cases: