Death's Shadow gets -X/-X, where X is your life total.
3/14/2017 Death’s Shadow’s ability applies only while Death’s Shadow is on the battlefield. In all other zones, its power and toughness are 13.
3/14/2017 The value of X changes as you gain and lose life. It’s not locked in as Death’s Shadow enters the battlefield.
3/14/2017 If your life total is 13 or greater and nothing else is boosting the toughness of Death’s Shadow, it’s put into its owner’s graveyard as a state-based action.
3/14/2017 In a Two-Headed Giant game, your life total is your team’s life total.
7/14/2017 If your life total is negative, X is considered to be 0. (This is a change from previous rulings.)