(Flagstones Tar Pit isn't a spell.)
Flagstones Tar Pit enters the battlefield tapped unless you reveal a Horror card from your hand.
All creatures have Plainswalk.
: Add or to your mana pool.
Sacrifice Flagstones Tar Pit: Add or to your mana pool.
: Add 
or 
to your mana pool. or to your mana pool.
