Players decide and announce in turn order, starting with the active player, whether or not they want to shuffle their hand and graveyard into their library
Along with Full Heal energy, this card was the first to provide a secondary effect in addition to energy, making it a Special Energy card
King Macar, the Gold-Cursed You can either build this as a card draw plus a Gold tokens machine looking to win with Disciple of the Vault, Marionette Master, and Revel in Riches, or you can lean into the tapping and untapping thing using your artifact synergies
The three mana is definitely awkward, as you won't be able to use it to protect a Violent Outburst as a Force of Negation would, but it does deal with more proactive answers such as Teferi, Time Raveler
The interplay of these factors creates a dynamic and often volatile secondary market
There are 54 inventions total in the Kaladesh block