From UBB Changes January 2010 Reset:
Assassin Slay ability now requires unit kills to activate.
Slay ability has been redesigned. It now lowers the stack kill percentage required to kill a hero. The formula is 20% + 5% per level. So a level 17 Assassin (level 4 slay skill) would have a 40% reduction to the needed kill percentage. In other words if they kill at least 60% of the paired stack then the hero leading that stack will be slain.