Kalisto, Vitality Leach

Double agent 4 (each opponent may pay {4}. Kalisto, Vitality Leach comes into play under control of target player who didn't pay this cost)
Kalisto, Vitality Leach enters the battlefield with X +1/+1 counters, where X is your life total.
When Kalisto, Vitality Leach enters the battlefield, your life total becomes 0.
You don't lose the game for having 0 or less life.
If Kalisto, Vitality Leach would take damage, instead remove that many +1/+1 counters from it.