Oni Curses are a feature in the Ninja Tower Event. When an Oni Stone appears on the board, it will randomly target a hero.
The player will have 3 turns to destroy the Oni Stone before it curses the targeted hero. If the timer runs out on the Oni Stone an Oni Curse will be applied to the targeted hero. Once a hero has been cursed 2 times, they can no longer enter Ninja Tower Battles for the remainder of the event. The number of curses on a hero cannot be reset, removed or dispelled and will remain for the entirety of the Event.
Once a hero has been targeted by a curse, the only way to avoid that hero being cursed is by matching the Oni Stone, before the time runs out. A hero who has been killed while being targeted with a curse can still get cursed if the gem is not destroyed.
A hero that has received their 2nd curse, will still be able to finish the battle in which the 2nd curse was received, but cannot be used in any subsequent battles.