For the effect of “Fatal Abacus” to trigger, a Monster Card that is being treated as a monster must be sent from the field to the Graveyard. This means that a Monster Card equipped to “Relinquished” or “Sword Hunter” that is sent to the Graveyard will not trigger this effect. Also, a Spell or Trap Card that is destroyed while being treated as a monster by the effect of “Magical Hats” will not trigger this effect.
Monster Tokens do not trigger this card’s effect because they are not sent to the Graveyard.
Monsters that are removed from play and monsters that are destroyed while “Banisher of the Light” is on the field do not trigger this card’s effect. This is because they never go to the Graveyard.
“Sent from the field to the Graveyard” includes cards that are destroyed, Tributed, or sent straight from the field to the Graveyard.
If multiple “Fatal Abacus” cards are active on the field, a player will take 500 points of damage for each when one of their monsters is sent from the field to the Graveyard. So if 2 are active, a player will take 1000 points of damage and if 4 are on the field (you have 3 active and your opponent has 1 active) a player will take 2000 points of damage.
It is the owner of the Monster Card that takes the damage, so if you control your opponent’s monster through the effect of “Change of Heart” and Tribute it, your opponent takes the 500 points of damage.