Magister sphinx

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.

This ability is not optional.

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.