As this land enters, you may reveal a Swamp or Mountain card from your hand. If you don't, this land enters tapped.
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The road appeared lonely and forsaken, running down to empty ruins in the mist. But there was an evil feeling in the air.
(08/04/2016)
Lands don't have a subtype just because they can produce mana of the corresponding color. Foreboding Ruins itself is neither a Swamp nor a Mountain, even though it produces black and red mana, so you can't reveal one to satisfy the ability of another.
(08/04/2016)
If an effect instructs you to put Foreboding Ruins onto the battlefield tapped, it will still enter the battlefield tapped even if you reveal a land card from your hand.
(08/04/2016)
If a Swamp or Mountain is entering the battlefield from your hand at the same time as Foreboding Ruins, you may reveal the other land to have Foreboding Ruins enter untapped.
(08/04/2016)
You may reveal any land card with either or both of the appropriate subtypes. It doesn't have to be a basic land. For example, you could reveal Sunken Hollow from the Battle for Zendikar set to satisfy the ability of Foreboding Ruins.
