2022-04-29 : As you cast a spell with casualty X, you choose whether to pay its casualty cost and what the value of X will be.

2022-04-29 : Casualty N means “As an additional cost to cast this spell, you may sacrifice a creature with power N or greater.” and “When you cast this spell, if a casualty cost was paid for it, copy it. If the spell has any targets, you may choose new targets for the copy.”

2022-04-29 : If you pay the casualty cost of a spell, the copy will resolve before the original spell.

2022-04-29 : The copied spell from Ob Nixilis, the Adversary's triggered ability copies exactly what is printed on Ob Nixilis, except its starting loyalty is equal to the chosen value of X and it isn't legendary. The copy becomes a token as it resolves.

2022-04-29 : The copy of the spell is created on the stack, so it's not “cast.” Abilities that trigger when a player casts a spell won't trigger.

2022-04-29 : You can control exactly one legendary Ob Nixilis, the Adversary and any number of nonlegendary copies of Ob Nixilis, the Adversary.

2022-04-29 : You may sacrifice only one creature to pay a spell's casualty cost, and you copy the spell only once.