(Horizon Vents isn't a spell.)
When Horizon Vents enters the battlefield, return a land you control to its owner's hand.
Green spells cost more to play.
: Add or to your mana pool.
Put Horizon Vents on top of your library: Add to your mana pool.
more to play.
: Add 
or 
to your mana pool.
to your mana pool. 
