Oni estertor de muerte

Count any creature that died during the turn, including token creatures, when calculating the cost reduction. For nontoken creatures that died, it doesn't matter what happened to the creature cards after they died.

The cost-reduction ability of Death-Rattle Oni doesn't change its mana cost or mana value, only the total cost you pay. Specifically, Death-Rattle Oni's mana value is always 7.