Sylvion: Below Ground

From BoardGameGeek:

Sylvion: Below Ground is a 16-card expansion that adds an additional challenge to Sylvion. Shuffle the cards, then place them face down in the 4x4 battlefield grid. During the game, when you want to place a tree or a fountain on a terrain card on the battlefield, you must first pay the cost, then reveal the terrain card. Then: If the terrain card is unoccupied, remove it from play and place the tree or fountain in this space. If the terrain card shows an owl, remove it from play, place the tree or fountain in this space, then draw two cards from the defender deck. If the terrain card shows a swallower, remove it from play along with the tree or fountain that you were going to place; you receive no compensation. (If the removed card was a fountain, draw one card from the defender deck as if the fountain were defeated in battle.)