Cain, The Shadow

Cain, The Shadow deals 5 damage to your
opponent's Life whenever one of your Assassin
Creatures deals a Critical Hit.
{3} [Unending Fatality] — You may use this
ability whenever a Critical Hit occurs. Put one
"Doom" counter on your opponent. During the
Upkeep, your opponent loses 5 Life for each
"Doom" counter they have on them.