Tashra, Fell Conjurer

When Tashra, Fell Conjurer enters the battlefield, create two 1/2 snake creature tokens
with "When this creature dies, you lose 1 life."
You may cast creatures with converted mana cost
X from your graveyard where X is the amount of
life you gained this turn minus the amount of life
you lost this turn. X cannot be 0. Activate only
once each turn and only as a sorcery.
{2} You gain 1 life.