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

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.

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."

You can control exactly one legendary Ob Nixilis, the Adversary and any number of nonlegendary copies of Ob Nixilis, the Adversary.

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

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.

If you pay the casualty cost of a spell, the copy will resolve before the original spell.