Cryptborn Horror enters the battlefield with X +1/+1 counters on it, where X is the total life lost by your opponents this turn.
10/1/2012 Remember, damage dealt to a player causes that player to lose that much life (unless the source has infect).
10/1/2012 If no opponent has lost life on the turn Cryptborn Horror enters the battlefield, it will enter with no +1/+1 counters on it and be put into its owner’s graveyard (unless something else is raising its toughness).
10/1/2012 Cryptborn Horror cares about the total life lost, not necessarily what an opponent’s life total is compared to what it was at the beginning of the turn. For example, if an opponent loses 5 life and then gains 10 life on the turn Cryptborn Horror enters the battlefield, it will enter with five +1/+1 counters on it.
10/1/2012 In multiplayer games, life lost the turn Cryptborn Horror enters the battlefield by opponents who left the game later that turn will count toward the value of X.