Unheilvolle Ruinen

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.

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.

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.

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.