(Obsidian Retreat isn't a spell.)
Obsidian Retreat enters the battlefield tapped.
, : Tap target artifact with converted mana cost X or less.
: Add to your mana pool. Return Obsidian Retreat to its owner's hand.
, 
: Tap target artifact with converted mana cost X or less.: Add 
to your mana pool. Return Obsidian Retreat to its owner's hand.
