Shaman of Forgotten Ways

• WWizards of the Coast2015-02-25
  Official Ruling
  If you control a creature with power less than 0, use its actual power when calculating the total power of creatures you control. For example, if you control three creatures with powers 4, 5, and -2, the total power of creatures you control is 7.
• WWizards of the Coast2015-02-25
  Official Ruling
  For a player’s life total to become a specific number, the player gains or loses the appropriate amount of life. For example, if a player’s life total is 12 and that player controls three creatures as the ability resolves, they will lose 9 life. Other effects that interact with life gain or loss will interact with this effect accordingly.
• WWizards of the Coast2015-02-25
  Official Ruling
  The mana produced by Shaman of Forgotten Ways can be used to cast face-down creature spells with morph or megamorph.
• WWizards of the Coast2015-02-25
  Official Ruling
  Count the number of creatures each player controls when the last ability resolves to determine what number each player’s life total will become.
• WWizards of the Coast2015-02-25
  Official Ruling
  Other formidable abilities are triggered abilities with an “intervening ‘if’” clause. Such abilities check the total power of creatures you control twice: once at the appropriate time to see if the ability will trigger, and again as the ability tries to resolve. If, at that time, the total power of creatures you control is no longer 8 or greater, the ability will have no effect.
• WWizards of the Coast2015-02-25
  Official Ruling
  Some formidable abilities are activated abilities that require creatures you control to have total power 8 or greater. Once you activate these abilities, it doesn’t matter what happens to the total power of creatures you control.
• WWizards of the Coast2015-02-25
  Official Ruling
  The first ability of Shaman of Forgotten Ways is a mana ability. It doesn’t use the stack and can’t be responded to.
• WWizards of the Coast2015-02-25
  Official Ruling
  If a player controls no creatures as the last ability resolves, their life total will become 0, causing that player to lose the game. If all players’ life totals become 0 this way, the game will be a draw.

Rulings via Scryfall