The Blackhand has the ability to take adjacent minions damage.
Some Mercs – like Diablo and Ragnaros would gain a passively treasure ability that “deal X fire damage to all enemies while an enemy is killed”.
Once Blackhand dies, there is an endless loop take place: the passive triggered → Blackhand gains the damages from adjacent minions → blackhand triggers the passive again → it takes the damages again… That loop keep running over and over, even when Blackhand became LESS THAN -1400 HEALTH…