(Howltooth Necropolis isn't a spell.)
Howltooth Necropolis enters the battlefield tapped.
: Target player exiles a card from his or her graveyard.
: Add or to your mana pool.
Sacrifice Howltooth Necropolis: Add or to your mana pool.
: Target player exiles a card from his or her graveyard.: Add 
or 
to your mana pool. or to your mana pool.
