Dahan Event: Forsake the Barren Land: On Each Board: Push 2 Dahan from a land with Blight to a land without Blight.
>> a board don`t have '2 Dahan from a land with Blight' but have '1 Dahan from a land with Blight'. can i triger '1 Dahan from a land with Blight'?
Tsunami : 2 Fear. 8 Damage. Destroy 2 Dahan.
>>target land have 1 dahan. 1 dahan is destroy?
Avoid the Dahan lv 1: Invaders do not Explore into lands with at least 2 Dahan.
>> if all land have only 1 dahan, event is nothing happen?
Forsake the Barren Land - I would say yes, if the only Blighted land with Dahan only has one Dahan in it, then you would just push that one Dahan, thereby fulfiling as much as possible of the Event text :).
Tsunami - Again I would say yes, destroy just that one Dahan. Think of the card as saying "Destroy as many Dahan as you can in this land, up to a maximum of two".
Avoid the Dahan - If all lands only have one Dahan, then none of them have two or more, so the Explorers can happily explore to their hearts' content because there aren't enough Dahan around to put them off doing so.