Cheapest priced copy (Streets of New Capenna Promos): $2.99 at Card Kingdom, near mint, their price list as of .
Ob Nixilis, the Adversary 
Legendary Planeswalker — Nixilis · Mythic · Streets of New Capenna · #206
Casualty X. The copy isn't legendary and has starting loyalty X. (As you cast this spell, you may sacrifice a creature with power X. When you do, copy this spell. The copy becomes a token.)
+1: Each opponent loses 2 life unless they discard a card. If you control a Demon or Devil, you gain 2 life.
−2: Create a 1/1 red Devil creature token with "When this token dies, it deals 1 damage to any target."
−7: Target player draws seven cards and loses 7 life.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
6 printings · $2.99 to $8.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Streets of New Capenna Promos | 206p | Mythic | $2.99 | $5.99 |
| Streets of New Capenna Promos | 206s | Mythic | — | $4.99Out of stock |
| Streets of New Capenna | 206 | Mythic | $2.99 | $3.49Out of stock |
| Streets of New Capenna | 284 | Mythic | $4.49 | $8.99Out of stock |
| Streets of New Capenna | 348 | Mythic | $4.49 | $4.99 |
| Streets of New Capenna | 448 | Mythic | — | — |
7 rulings from Wizards of the Coast
- 29 April 2022As you cast a spell with casualty X, you choose whether to pay its casualty cost and what the value of X will be.
- 29 April 2022The 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.
- 29 April 2022Casualty 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."
- 29 April 2022You can control exactly one legendary Ob Nixilis, the Adversary and any number of nonlegendary copies of Ob Nixilis, the Adversary.
- 29 April 2022You may sacrifice only one creature to pay a spell's casualty cost, and you copy the spell only once.
- 29 April 2022The 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.
- 29 April 2022If you pay the casualty cost of a spell, the copy will resolve before the original spell.