Magister Sphinx

• WWizards of the Coast2009-02-01
  Official Ruling
  This ability is not optional.
• WWizards of the Coast2009-02-01
  Official Ruling
  For a player's life total to become 10, what actually happens is that the player gains or loses the appropriate amount of life. For example, if the targeted player's life total is 4 when this ability resolves, it will cause that player to gain 6 life; alternately, if the targeted player's life total is 17 when this ability resolves, it will cause that player to lose 7 life. Other cards that interact with life gain or life loss will interact with this effect accordingly.
• WWizards of the Coast2010-06-15
  Official Ruling
  In a Two-Headed Giant game, this ability basically causes the team's life total to become 10, but only the targeted player is considered to have actually gained or lost life.

Rulings via Scryfall