Death-Rattle Oni
Creature — Demon Spirit
Flash
This spell costs 
less to cast for each creature that died this turn.
When this creature enters, destroy all other creatures that were dealt damage this turn.
Wayne Reynolds
Rulings
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.
(wotc • 2023-05-12)
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.
(wotc • 2023-05-12)
Rulings
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.
(wotc • 2023-05-12)
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.
(wotc • 2023-05-12)